Software Estimation: Demystifying the Black Art (Best Practices (Microsoft))

Count if at all possible. Compute when you can't count. Use judgment alone only as a last resort.

Look for something you can count that is a meaningful measure of the scope of work in your environment.

Collect historical data that allows you to compute an estimate from a count.

Don't discount the power of simple, coarse estimation models such as average effort per defect, average effort per Web page, average effort per story, and average effort per use case.

Avoid using expert judgment to tweak an estimate that has been derived through computation. Such "expert judgment" usually degrades the estimate's accuracy.