There are many answers to this question. But let me ask a few questions:
1. How experienced are you with mixing and mastering? 2. What kind of music are you mixing: What instruments are in the mix, and how many tracks? 3. How much do you know about compression? 4. How much do you know about equalization? Are you familiar with low & high pass filtering, and notching? 5. What plugins/effects do you use? 6. Do you know how to use FX channels and busses? 7. Do you know how to deal with frequency collisions? (this is when two instruments occupy a lot of the same frequency ranges, and wind up masking each other).
Sorry if you already know all about this stuff, but I'm assuming you're new to mixing because the question is a very general question. If you post some detail about how you're going about things, you'll probably get lots of helpful responses. Good luck!