...use an EQ to cut the low frequencies (up to as high as 250Hz) in every track, save the bass drum and bass guitar. Cut them too, but only up to 60Hz maybe.
You can find spectrum analyzers that will help you see where frequencies are piling up, but you should mix with your ears. Start low cutting your tracks, and see how that changes things. Also, don't solo your tracks while you EQ - you likely won't like the sounds of your tracks while they're soloed, but they'll fit better in your final mix.
To get some definition on the kick drum, I found the fundamental frequency of the 'click' of the drum beater, and boosted that a bit. I found a boost around 100Hz helped too.I cut the bass guitar in about the same 100Hz region too.
Listening to [your] mixes in a great room and fabulous speakers is a revelation...
...as a general rule I only leave the kick, snare, bass and lead vocal in the center. Only on very rare occasions will anything take over that coveted real estate. Often I'll pan things hard right and left.
If there is a sound (like acoustic guitar or backing vocals) that I want to position so that it is audible in both speakers, i'll often use a stereo delay set at 100% wet, where the left delay is set to 0 ms (no delay) and the right is set to somewhere between 5 and 15 ms. Just that little shift is enough to throw the sound to the sides and leave the center clear for the most essential information, while being virtually indistinguishable to the ear.
Use reverb sparingly. Too much (or poorly eq'ed) reverb makes for all kinds of mess.
Get a hold of a spectrum analyzer, sometimes called a Real Time Analyzer or RTA. I use Voxengo GlissEQ which is just an amazing eq in itself, and is also a fantastic SA. [...]
Try a limiter, multi-band compressor or sonic maximizer. Be careful because it's easy to overdo it and squash the crap out of (or maybe into?) your music...
...Listen to sgt pepper on your monitors over and over again.
Mix at lower volumes. really low even. definately helps with vocal and bass placement and ear fatigue.
Occasionally switch to mono to make sure nothing is sticking out of the stereo field too much (or disappearing).