Record your impressions. Determine if you had a positive or negative impression about the people you met and the position for which you interviewed.
Evaluate yourself. If there was "one more thing you wish you had said," try to incorporate that into a thank you letter if you decide it is appropriate to send one. If you felt that an interview went badly, try to figure out why. If the reason is within your control, change your interview behavior to prevent the problem in the future.