Generally it is best to try and master on a per track basis if possible. That way you will end up with fewer audio issues in your mix that need to be fixed by a professional.
Do your best to try not to 'master' your track until you are finished your song...
To do this... in live or in another application... I put an eq on any track that needs to be tamed, enhanced etc... if you have a really hard time boosting a track, try running it through a tube for gain or use a compressor to increase the overall volume of the track.
Personally I try and stay away from compressing the master track because you will very likely kill the dynamic volume of your song... ie everything will be loud.
If you are at all skilled as a dj then you'll already know this, but say you have a snare track and drum track that has more snares in it. if you want to enhance one set of snares over the other you could eq one track to emphasize the sound for one, and de-emphasize the other. This can be applied for drums... leads... pads... anything that you need to chop away at just enough to give room to another sound, but to still keep it in the mix.
Now... having said all that... if you get good at it, and listen to your mix, you can ride your eqs to dynamically shape and move your song on a track per track basis. Listening, you should be able to have a good idea of which part of your song needs to take the lead.
One last 'trick'... I like to have a reverb effect on the send track, to make sure that any track that doesn't have reverb on it, can be added to the 'room'. Quite often you'll find that a track, or sample sounds out of place, like it is someplace else. If you just squeeze a little reverb into it, then you'll get a bit more presense and grounding for the sound. You'll find this to especially be a problem for samples that are entirely synthetic...
Good luck!
