I've never done it.. but have read the process many times.
1. get a lot of jackstands.
2. RAISE the STERN of the boat by lowering the trailer jack.
3. slip a number of jackstands under the STERN of the boat.
4. RAISE the BOW of the boat by raising the trailer jack.
5. slip some more jackstands under the bow.
Work the trailer out from under by alternately raising and lowering the
trailer, and inserting and re-inserting the jackstands as needed to slip the
trailer out.

Candidly, the thought of any one of the jackstands slipping or not being
positioned properly is enough to make me wait until Summer and put her in
the water....but...there you have it.
--
RichG
http://groups.msn.com/CarolinaSkiffOwners