My friend Johanna Rothman just mailed out the latest issue of her newsletter, The Pragmatic Manager. This issue had good advice about reducing the length of development tasks to what she calls "inch-pebbles."
If you estimate in days, you'll be late by days. If you estimate in weeks, you'll be late by weeks. ... If you estimate in months, you'll be late by months.
I also liked this quote from from Johanna:
The schedule is the one way the project won't work.