I find it interesting to try to figure something out and then check it against an accurate tab to see what I may have missed or got wrong. Even if you think a song is beyond your grasp to figure out by ear, give it a try first. You'll learn more from it when you go to the tab.

To figure out chords, I'll use the bass strings or power chords to sort out the chords. Then figure out major/minor/seventh/etc. Here is where some theory knowledge can help because it narrows things down and you aren't just shooting ramdomly in the dark.

Another things that's fun to do is have your guitar at hand while watching TV. Try to figure out what key a show's theme song is in, figure out the melody, etc. Do this with commercials too. You're not doing this so you can play the song later, it's just an exercise in training your ear and being able to learn a song by ear and also play by ear. I remember being in a guitar store and a kid was playing the theme from the "Ren and Stimpy Show".