I consider Mike Cohn one of the best on the topic of Agile estimation. His 90 min presentation on how to estimate is well worth your time. I took his one day course and all of the material in the presentation appears to come from that course.
Mike is truly inspired to find the best ways to estimate. He once forced each of his teams to pick a different method of estimating, too see which would win out a la Estimating Survivor.
Learn why relative estimating is better than absolute, how too much information can lead to poorer estimates and get good at (planning) poker plus a whole lot more.