The Rules of Agile Estimation:
http://railsrx.com/2010/05/13/may-13-2010-the-rules-of-agile-estimation/

1. Estimates are always wrong

2. If you think spending more time on estimates is a good idea, see rule 1.

3. On average, an experienced developer is not going to improve on his
or her gut reaction by thinking it over.

4. Team estimates are important, one person may see something that
everybody else missed. Just keep it quick.

5. People are much better at estimating size relative to each other
than absolute time a task takes.

6. Separate the problem into smaller chunks, the more estimates you
make the better the chance that the law of averages will help you.

7. Decomposition into roughly equal sized tasks is pretty much the
whole ballgame.

Alex N.