With everything going on out there in the great big world (including Ari Hoenig's great band on the live feed from Smalls) I thought I'd just mention a couple of things I do to help me listen and concentrate, especially before gigs.
One of the main things I do is to listen to music, any music (the radio will do nicely) and focus on what is usually the most difficult thing to hear, the bass. I also:
1) Try and predict where it's (the bass line or root movement) going to go next,
2) Sing along. You don't have to sound like an opera star, but try to be as accurate with the pitches as you can. This helps everything!
3) Notice the relationship between the bass and drums. If it's a Pop, Rock, or R n' B tune do the drums play rhythms in unison with the bass, or do they both go their own way? If it's a Jazz tune, do the bass and drums play the time in the same place (in the middle, top, or bottom of the beat?) or in different spots?
I find doing some of these things helps my ears (which at this point are the part of me that I worry about my chops the most) and gets me ready to listen.
To close here's a great example of Bass and Drums playing counter to each other in a tune.
Bowie's classic "Ashes to Ashes". Very cool video too!