The basic idea is to take the "main" frequency range of one instrument and cut that from some of the others to give it space. An obvious example is your bass - you don't really need the guitars to have much low-end with a big, spankin' bass playing along, do you? By applying a low-shelf, or a high-pass, or just a wide band cut around 80 or 100 Hz on your guitar tracks, the bass has more room to play with. A plugin like Voxengo Span (it's also free) can be really helpful in figuring out where the body of an instrument/voice is.