If there's any consolation to the mistakes the U.S. has made in the past in its efforts to overthrow governments, I can point to two recent examples of GOOD ways to support positive democratic movements: The Rose Revolution in Georgia about a year ago, and the Orange Revolution in the Ukraine at the end of last year. A lot of U.S. money backed these groups of young people who wanted genuine democracy in their own countries, and that money went a long way in helping train these young people in the ways of nonviolent protest. As a result, two governments have fallen and been replaced by more liberal democratic ones.
This seems to be the best way to do so. Maybe it could also work in Iran. There are certainly a lot of young Iranians who want the same kinds of freedoms we take for granted here, and who have to lead "double lives" of sorts to get by: the life they lead at home, where they wear jeans and Western-style clothing and listen to rock & roll/hip-hop music, and the life they lead outside, where they have to wear veils and such.