Good tips here. I’ll go into a bit more detail offering my insight as a composer. Starting with a handful of sample libraries can make things difficult in the quest for realism, but there are a number of things you can do to make your life easier. Most of this deals with strings, but some of the techniques can be applied across the orchestra.
None of these tips require you to buy new libraries, except obviously the ‘recommended libraries’ And No, for the record, I’m not affiliated with any of them. 
VELOCITIES
This is by far the most common problem when hearing tracks that are produced with samples, especially in 'epic' tracks. A lot of composers like to just hammer the keys at the highest velocity the whole time, and that makes it 'epic'.
Well, actually it doesn’t. What makes things ‘epic’ is the builds and swells. And to do that, that requires a degree of performance. Even the cheapest libraries have articulations recorded at multiple velocities, so it’s best to make use of them. If they softer velocities are not loud enough than use the volume controls to compensate.
If you’re triggering the hard velocities for short notes the whole time, it just sounds static and repetitive. Static and repetitive is exactly the opposite of what a string section is.
If you listen to Hans Zimmer’s ostinatos, which let’s face it, is what a lot of us are trying to emulate, there is ALWAYS subtle variation throughout.
RIDE THE MODWHEEL/EXPRESSION CONTROLS
Most of the sustained/long patches on string libraries have some kind of mod wheel control nowadays. If not you can always program this yourself (expression, or volume control inside Kontakt). It's best to make use of what the developers have made for you though.
The most important lesson I learned when doing mockups is to CONSTANTLY be riding the modwheel. There is no such thing as a static string sustain in real life, and that is what the vast majority of libraries are doing right now, they record a long sample, and then loop it.
I’ve heard a lot of tracks, particularly those that are either doing epic works, or the romantic/sentimental tracks, where long sustain samples are used, and the biggest weakness is that none of these sustains have any movement or life.
You don’t need to go overboard with this, but you do need to do ‘something’ to give your static samples more life.
LAYERING
This is one of the easiest ways to hide old/bad/cheap samples in my opinion. And it is something that is a necessity when some of your parts are exposed.
Recently I’ve heard a few tracks where some sampled strings are playing by themselves, and the samples are of not particularly high quality. You need to hide that! And it’s easier than you might think.
The hallmark of bad samples is usually in their ‘attacks’, i.e. how the instrument enters. However, if you have no alternative, your best option is to hide the attack.
For example, if you are writing a soft, sentimental part with string chords, why not accentuate it with light harp arpeggios when the chords change? This will 1) add flavour, and 2)hide the samples
Another example, if you are playing a long sustained legato line, but your legato samples kinda suck, then why not help it along with another texture occasionally. For example, violins+flute work well, violas+french horn. Instead of having cellos and basses play in octaves the whole time, have the cellos play legato and have the basses play pizzicato an octave lower (a classic Ravel trick). Use piano or melodic percussion to double the melody.
Use your good samples to help out your bad ones.
Follow tried and tested and centuries old orchestration techniques to help your samples. You’d be surprised at how easy it is! It does require effort and a bit more time though. But once you get into the habit of doing these things, it becomes second nature.
There are plenty of orchestration books that can help you with good sound combos, but an underrated online resource is the VSL Academy.
VSL Academy - http://www.vsl.co.at/en/70/149/150/46.vsl
Example of sound combos - http://www.vsl.co.at/en/70/3189/3190/5626.vsl
Additionally, you can buy film scores from Alexander Publishing and study them. You would be amazed at how much doubling happens in a film score. All of these instrument combinations are used by the pros, you should use them too. The resources are available for you, use them!
Alexander Publishing - http://www.alexanderpublishing.com/Departments/Film-Scoring-and-More.aspx
REVERB
Most of this has already been covered in the posts above.
Quite a few composers obsess about getting things ‘sounding like they are in the same room’. Reality check - no-one cares as long as it sounds good. The amount of reverb and processing that goes on about 90% of film scores other than perhaps Michael Giacchino and John Williams today means that a bunch of rooms are being piled on top of each other.
You do however, need to add reverb to your strings, no matter how many libraries you are using and you need to give them their own space. Additionally, generally with bad samples, you can hide their badness by using a good reverb.
For orchestral music, your reverb unit is one of the most important choices you make. It really can bring life to your productions. If the decay/release sounds digital DON’T USE IT. You want something that more than anything else sounds like the sound is dieing away naturally. Use your ears. QL Spaces is generally a good starting reverb unit. I personally live by 2CAudio B2 and Aether now.
QL Spaces - http://www.soundsonline.com/Spaces
2CAudio B2 - http://www.2caudio.com/products/b2
2CAudio Aether - http://www.2caudio.com/products/aether
Do NOT skimp on your reverb if you are doing orchestral music. There is a reason that the top film scores are using $7000 hardware reverb units, it’s because reverb is important!
How you apply the reverb is generally a creative decision, but a good reverb will help add life to static samples, thus it is an important decision on which one you choose. Most reverb plugins have trials. Use them! And then, use your ears!
ARRANGEMENT / VOICE LEADING
Believe it or not, if you write for strings well, and follow some general rules of arranging and voice leading, they are probably going to sound good in your samples too!
A mistake a lot of composers make when starting out is to play string parts in like they are playing piano.
A string orchestra is NOT a piano.
Study scores (as mentioned above) to learn how string writing works. Better yet, try to listen to scores and/or pop music that uses strings and try to do a dictation of it.
Pay attention to the harmonic series when writing for strings. Do not write bass notes close together unless that is a deliberate muddy effect you are going for. Generally speaking close voices at the top, and more open, wider voices at the bottom. (But there are all kinds of exceptions). As always, use your ears.
USE PERFORMANCE SAMPLES!!
By this I mean, make use of samples that aren't static. Some libraries have prerecorded crescendos / diminuendos. Use them! The tremolos in most libraries are incredibly useful because those samples aren't static! Pre-recorded string runs are not static, thus they sound good!
Any sample which captures a ‘performance’ is INFINITELY better than a sample which is static.
I’m a huge fan of 8Dio’s approach in this respect, as their Adagio series - while flawed in many respects - capture this concept better than any other library. They are at least trying to do something new, especially with the ‘long’ sounds.
MIXING/BALANCING
Graham already mentioned this. Try to send your tracks to different groups. You don't necessarily have to go as far as Graham did, but you can. I prefer to just keep things to 'High' and 'Low Strings', and 'Long' and 'Short'. That is my style of writing though. Do whatever suits you.
It’s important to keep your mixes organized so you know what you’re changing.
As for the actual mixing and balancing, the usual problems with sampled strings are frequency buildups.
When you play a triad on the violins for example. You’re not playing 16 violins spread across 3 notes, you’re playing 16 violins + 16 violins + 16 violins, thus 48 violins! This is not natural. This is why you should try and write lines separately when possible (there is not always time). If you don’t have time to write separate lines, then you need to EQ to stop the frequency buildups, because they are unnatural.
In general, they occur in 2 areas. 1) Around 300 Hz (this is generally a problem area). 2) In the high end, particularly at 3000 Hz.
Additionally, if you are writing for violins, you don’t need low-end frequencies! They add absolutely nothing! Vice-versa for cellos, you can cut some of that high-end information, it’s not going to add anything.
Some good sample libraries have already done this for you (Adagio), but many leave the room’s noise in there (Eastwest) which unless you want the noise (sometimes you do) you need to filter it out anyway.
You don’t need to be to aggressive when EQ’ing your strings, but if you are using EQ, you should be cutting, not boosting.
MASTERING
I'll keep this brief. Please do not add tons of compression to your orchestral only tracks. Even the hybrid stuff. It's just so unnatural! A common thing I've heard a lot of is applying compression onto orchestral tracks after the reverb has been applied. This compresses not only the samples but also the reverb too! It does not sound good.
Simply put, compression on orchestral work, should not be necessary and if it is, it should be applied in a subtle way. You should only be pushing the volume/limiter a little bit, the rest should all come from the mix.
If you’re doing hybrid work and need your spiccato string samples up front, then by all means, compress. But apply the process BEFORE you add your reverb, otherwise you will just be compressing the reverb too and it will sound like a mess.
RECOMMENDED STRING LIBRARIES
GENERALLY speaking, you should look to have one library that handles 'ensemble' sounds, and one that handles 'detail'. Sometimes that ensemble sound where all the players are in the same room palying at the same time just has a different 'feel' than the libraries where sections are recorded separately.
Having that ensemble ‘accompaniment’ sound can be incredibly useful, and then you add a solo line on top (such as a violin line) can really make things shine. It does depend on your style of composition.
ENSEMBLE RECOMMENDATIONS
8Dio Adagietto.
Not strictly an ensemble library, (it’s made up of each section from the Adagio library) but it does a very good job of impersonating an ensemble sound. Very good value.
Spitfire Audio - Albion 1/2
Albion 1 in particular is a very good ‘ensemble’ workhorse.
Symphobia 1
I was one of the first people to purchase Symphobia. 6/7 years later I still use it in almost everything I do. The ensemble sound they have is second to none in my opinion. Paired with a good reverb, this library is tremendous.
Not a fan of Symphobia 2 (doesn’t have the magic of the original), and don’t use Lumina, though I’m sure they are both decent.
I’ve also heard good things about Cinematic Strings and Da Capo, but do not have personal experience with them.
DETAIL RECOMMENDATIONS
8Dio Adagio
For reasons mentioned previously. They simply have more ‘performance’ samples than anyone else. If you take the time to learn this library, you will get tremendous results.
LA Scoring Strings
Not for everyone, and has a very clunky interface, but has a very intimate sound, that once tamed with EQ, can bring tremendous depth and richness.
I’ve also heard good things about Berlin Strings in particular, and Eastwest Hollywood Strings, but don’t have personal experience with either. Hollywood Strings in particular has a huge amount of ‘performance’ samples.
Cinematic Strings also has single string sections as well, and might be a good bet in the ‘detail’ area too.
These detail libraries are expensive. Generally go for the ‘ensemble’ libraries first, and then expand when you can.
CONCLUSION
Obviously it's going to be impossible to do all of this stuff at once, but try and incorporate some of this stuff into your work flow. The most important thing to remember is to make sure your string writing is actually good string-writing and not piano writing, and then to make sure that your samples do not sound static! Then do the production stuff (reverb, EQ) afterwards.
You’ll generally find most of the problems with your samples come at the compositional stage, rather than the mixing stage. However, once you get good at both, you can get incredible results.
Hope this helped!
