First, use a tripod made of wood. (Carbon fiber and basalt/fiber glass are better than metal.)
There are the Meade vibration isolation pads:
http://www.astronomics.com/main/prod...product_id/895
Read paragraph 2 of this article for some typical ways to dampen ground vibrations:
http://www.pietro.org/Astro_C5/Mods/tripod_tray.htm
Here are some ideas:
http://photo.net/bboard/q-and-a-fetch-msg?msg_id=00BXbT