Until you're really aware of what compressors are doing to the sound you're putting through them, you may want to start off with extreme ratio and threshold settings and back off from there.
Try this out... increase your compression ratio as high as it goes. Then lower your threshold as low as it goes. Keep the attack and release as short as possible, and leave the makeup gain at unity for now. Now put your drum track through it (not a submix, just one track for now). You shouldn't be hearing much at all becuase of the low threshold. Increase the attack time slowly and you'll start to hear the transients (mostly high frequencies) snapping through. Increase the makeup gain carefully and you can hear it more clearly. When you adjust the release, you can prevent or induce "breathing", which is the sound of the compressor returning your signal level to normal after the signal drops below your threshold setting.
Aside from creative use, you would typically want to make sure your release time is short enough to not overflow onto the next drum hit. That means the compressor will not be active when the next transient comes around, and the attack of the next hit will trigger the compression again. If it's still active (ie, your release time is too long), the level of your next transient will be lower, and you may get somewhat inconsistent transient levels.
Try raising the threshold (you may have to lower your makeup gain if you've increased it a lot) and you'll get more of your original sound through since the compressor will be activated less often, and will perform less gain reduction.
When you know what you're listening for when using a compressor, you'll be able to utilize it to massage things in or out of the track. Another way some mixers use them is to set a moderate ratio (maybe 2-5), set the attack and release times according to the material (fast attack to kill a transient, slow to let it pass through, fast release to minimize the artifacts, slow release to sculpt the sound after the transient), and then lower your threshold until your meters show moderate gain reduction (program dependent, but i'll say that 3dB of reduction is a good starting point for creative compression). Note that this approach is surely for those who know the sound of the compressor they're using. Anytime you're mixing with the meters instead of your ears, you can get into trouble.
But really, compressors are tools like all other things. Is your drum track not a consistent level? Use a compressor to even out the performance. Apply the threshold so it tames the louder hits and leaves the softer hits alone. Is your snare muddy and dull? Use a compressor to let the snap of the transient through while attenuating the rest of the hit. That contrast will make the snare pop more.
Oh yeah, use makeup gain to bring your signal back up to where you want it after squashing.
Good luck,
marcus
