What Are The Keys to Measuring the Effectiveness of Individual Teachers and Principals?

Colorado passed SB191 (pdf) which requires the state and school district to come up with new (and presumably better) ways to evaluate teachers and administrators. Here is the question that my district is currently trying to answer:

What do you believe are the keys to measuring the effectiveness of individual teachers and principals?

I put the question out on Twitter and got quite a few good responses, but I thought I'd post it here as well to make it easier to post a more-than-140-character-at-a-time response. Please keep in mind this is not theoretical, my district has to come up with and implement this plan, and it will be used for evaluating teachers and administrators.

I'd love to hear your thoughts.