The best I've found is to use MS Flight Sim 2004 and download this
cross country "thermal" program (got to keep current on these long
nights):

http://x-c.home.att.net/

Be sure to follow the installation instructions and adjust the weather
and time of year for good soaring conditions. You can also adjust the
strength and max altitude of thermals.

Then download some great ships here (I've been flying an ASH25 through
the rockies):

http://www.fsglider.de/frset_e.htm
http://www.fszwever.com/