One of my favorite pieces of advise { I think I heard Pete Seegar say this } is that you should always end your rehearsal time playing something you can do well. That way your last memory of playing your instrument is a good one.

Also - A lot of people use way too much energy in the chording hand. Learn how to play the chords so that you don't wear out your hand. Play firmly with the right hand and lightly with your left. Assuming that you're right handed.