I agree with tehplatypus. You need to make sure that your master's meter isn't going into the red. It wont be very(if at all) noticable on playback, but once you mix it down as a .wav, you'll hear all the clipping.

Also, if you're using compressor's, make sure you're not going overboard with them. Use effects at a bare minimum. Too much reverb and such can really muddy up a mix, and give it a real "processed" sound to it.

One more thing. Are you panning your guitar's, bass, drums, vocals, etc... so they're not all right on top of each other?

You also might want to take a look at this thread. It has some info on eq'ing your mix, so everything fits into it's own sonic space.