I estimate by considering an optimistic, pessimistic and normal value for each task. I then have a spreadsheet that does a modicum of statistical analysis to give you times against probability that the task will be finished.

Some of the benefits I find are:

1. Getting into the pessimistic mindset helps produce better estimates

2. Tasks with a large spread are things you aren't really clear about, and are an obvious target to be attempted first

3. Giving a range of values to the person asking for the estimate seems to help them remember it is an estimate.