DISCLAIMER: I'm not a pro engineer, but these are tips I've picked up over the last 7 years.
Mixing is a LIFETIME'S study. Many people don't realize this. The best engineers in the world have generally been around studios for a very long time. I'm going to comment more from the orchestral side of things but many of these ideas apply across all genres. These are all things I've picked up having been lucky enough to hang out with some super talented people!
That said, there are some simple rules you can follow that will immediately help your mixes.
It is far better to cut EQ (reduce frequencies) than raise (add gain) to frequencies. By cutting things out, you're giving more room for other frequencies to shine. In general, CUT FIRST, raise second.
Do 'obvious' EQ! If you have a flute playing, why do you need anything below 100 Hz? You don't. You are always looking to cut the things that aren't adding anything to your music.
Avoid low-end build up. This is related to the first note. When we play an instrument it resonates at multiple frequencies, and if it's live / recorded in a room, it will pick up the characteristics of that room too. This can lead to an unwanted buildup of low-end. This is a very big problem with the majority of sample libraries - although recently some companies have started EQing out all the low-end stuff (8Dio Adagio is a good example, do a frequency analysis on the violins, and you will not find any low-end whatsoever). Most people's sample-based orchestral work - including my own - has way too much low-end info, and as a result the mixes can sound pretty muddy, so this is something I always tend to look at first. If you can get the bass (the foundation) of your track right, it's quite hard to go wrong after that as long as the bass isn't over-dominant.
Notch filtering to find unwanted frequencies.
When you have a lot of instruments playing in a room - or even a bunch of different samples playing together - you get a lot of reactionary frequencies that occur that can lead to a buildup in harmonic overtones that you don't want in your music. This is natural physics occurring, so there is nothing you can do about it. To find those unwanted frequencies, I load up a Notch Filter in my EQ program and do a frequency sweep to zero in on the frequency that is bothering me. A notch filter is simply an EQ with an exceptionally high 'Q'. It looks something like this.
By clicking on the headphones (this is Fabfilter Pro-Q) - I can isolate what the notch frequency is filtering out and then drag the headphone along the frequency line to listen for other unwanted frequencies.
This technique is particularly useful for precision instruments, but also good in general to find unwanted frequencies.
Panning. This is pretty obvious, but it's especially important in orchestral mixes. Listen to orchestral recordings you're trying to emulate and work out where the instruments are. It is one of the easiest things you can do to add space to a mix.
Reverb. Most of the best film score mix engineers heavily use multiple different kinds of reverbs in their work. (I have seen this first hand from Dennis Sands and John Rodd). You're trying to put instruments in their own space and give them their own control. Generally I'll use a different reverb for each section. One for brass, one for high strings, one for low strings, one for woodwind, one for percussion.
NOTE that they don't have to be different ROOMS for each section, I just like to be able to set the 'Dry/Wet' ratio for each section independently. You might want really lush strings (big reverb) but you don't want them to be muddied out with those warm brass chords also playing through a big reverb - so you put the brass chords through the same room but with a higher amount of 'Dry' signal, to give things a bit more clarity.
EQ'ing reverb. As always, try to filter out the low-end with reverb, otherwise it can cause havoc with your mixes.
A good reverb for orchestral work is Quantum Leap Spaces. http://www.soundsonline.com/Spaces
I'll also throw my support behind 2CAudio B2 - a phenomenal algorithmic reverb. http://www.2caudio.com/products/b2
That's about all I will offer for now. The most important lesson I've always tried to remember is CUT, CUT, CUT, when it comes to EQ. If you feel like you're losing volume, just...turn up your master fader. By cutting, you're going to naturally add clarity. By boosting, you're accentuating an instrument's natural sound, and therefore making it less natural. By cutting, you're only taking away info that we don't need to hear anyway.
Cut lots, raise little, and things improve very quickly
@tacoMusic - as for stems. Your suggestions are good, but I would try and separate Violins 1 & 2 if they have independent lines. Also try and separate a stem out for solo instruments if there are any. In general, the more stems the better, but obviously use common sense, as every track is different.
This is the stems I render when I do mixdown my sequence. I used to bounce all of these individually!! But now I finally bothered to set up a good routing system that when I do a mixdown, it renders not only a rough 'master' mix but also every stem at the same time.
Probably overkill for this purpose, but you may find it useful for reference. If you want that big punchy percussion sound that you mention (thanks for the shout out!) - then you DEFINITELY need to separate the percussion into low/mid/high.
Hope this info helped a little. This is a good thread.