I asked George Cables this question once, quite a few years ago, and what he told me stuck in my mind ever since. There is only one reason to comp and that's to make the music better. If you're comping just to hear yourself, then sit on your hands. However, if you feel like the music needs a little something, if you feel like the soloist needs something to support what they are doing or if the bass player is moving harmonically and you're digging on it, then enhance what's going on with your comping.

If you're just beginning, then comping can be a real mystery. Sometimes it's best to comp with a rhythmic idea in mind. The easiest one is the rhythm behind the old song, "The Charleston". That rhythm is one of the most well known figures out there.

Check out James P. Johnson playing "The Charleston". After he gets through the intro, the rhythmic figure stands out in the rest of the song. You can comp with this rhythm when you don't know what to do.

Another trick is to listen to the drummer. The drummer plays little hits on the snare and other drums/cymbals that are off-time but fit right in. Try to fit your comping into that. You don't want to comp 1-2-3-4 or always on the 2-4 or 1-3.

In the end, just try to make the music better, and the way to do that is to listen carefully, objectively, as if you are in the audience. If you need to record a rehearsal and listen to it, then do that (it's the best way to find out where you need to grow anyway!)

Good luck and let us know how it goes!