This is what I learned this summer, but I haven't really had a chance to release new tracks using these techniques, yet. But this might still help you.
Get a good EQ plugin, and learn how to use it.
Cut, cut, cut, cut, cut. NEVER boost.
Think about it in terms of frequency space, and what's supposed to occupy that space.
Your melody instruments, and things like snares and cymbals have NO business in the low frequencies, but sometimes samples have extra crap down there. That extra crap will mud up your overall mix, and it will also cause it to clip sooner. SO. CUT it out. The ONLY thing that should occupy the frequency range from ~30-200 is the bass, and the kick. Nothing else. But, like I said, sometimes samples and VST's will have stuff going on down there that really isn't 100% essential to the song. Hell, it might not even be noticable behind the bass/kick, so just get rid of it.
Also, cut out the super low and super high. Human ears can't ear them, but the technology can. If you have frequencies happening that you can't hear, but they ARE still there, it WILL cause your mix to clip and distort before you can make it as loud as you want it to be. So cut all that out. Anything below 20HZ is inaudible, so you don't need it in your mix. The same applies to the super high frequencies.
Also, if it's muddy, murky, and a bit muffled, cut out ~150-300 in all tracks. Just give it a try. It might make everything much, much clearer.
After I do all the EQ junk, and get levels/panning the way I want it, I dorp everything so I have about 5-6DB of headroom, and I bounce that. Without normalizing. Now I have a quiet, bounced wave file that I can work on in the mastering stage. The extra room on top means there's more space to crunch it in with mastering tools, like compressors and limiters... which I'm still trying to figure out.
But I can definitely say that, since finding a good EQ plugin and really learning how to use it, my mixes sound much better.
Again, EQ is all about cutting out what's unnecessary. On each track, for each instrument, find out what small frequency range is essential for that track, and cut out everything else. Less unnecessary frequencies = cleaner mix = LOUDER final product without clipping and distorting.