As far as applying compression goes (I assume you're talking about compression on a track by track basis rather than the multi-band compression used for mastering) you have to learn how to hear its effect.
Solo a track and apply some compression. Whack up the ratio to 8:1 and make sure the threshold is such that you're getting some regular gain reduction. Now with the release time set to something like 500ms adjust the attack time starting at 0ms. You should hear the sound change as you do this whenever the compressor is giving you some gain reduction. When you get it sounding how you like it stop. Now do the same with the release time. Then do the same with the compression ratio. Then adjust the threshold so you get as much or as little gain reduction as you like the sound of. This way you should hear how each aspect of a compressor changes the sound.
I use different settings for every song that I record because every song is different and every mix is different. I use the compression settings to serve the song.
My general rule of thumb is that if I can hear the effects of the compression I'm using too much (unless that was my intention).
It helps to have a very transparent sounding compressor if you're using it on lots of (or all of) the tracks in a song.
Pete.
