How can one learn from poor teaching evaluations?
Ben Norris's answer is excellent but I would add one additional point to it. When you are asking for constructive feedback from the students, you must do so in a way that students feel completely comfortable that their honesty is not going to come back to bite them.

At the end of every semester, I email all of my students a web-based survey with some open and some closed questions specifically so I can get their honest opinion. Again, the key is that all responses are anonymous. I believe if I asked them to email me (not anonymous) I would get nothing but praise, which does not help me improve at all. While I do still get many positive comments, there are usually some small gems in there which help me improve.