Exploiting the Existing Iranian Seam
The Strategy Page has an excellent assessment of Why Iran Can't Control Itself. After setting the pretext of Iranian funding and influence of the dominant Shi'ite Iraqi Dawa and SCIRI parties and the Iranian-backed militias in the south of Iraq, the following is an excellent glimpse into what both holds Iran together and will eventually lead to the demise of the mullah regime:
Diplomatic pressure on Iran has failed, because Iranians admit that the trouble is being fomented by Iranian Islamic radicals that the government cannot control. Iran is run by a coalition of Islamic conservative groups. They stick together because they represent only about a third of the population, and know that openly feuding could lead to losing power. This coalition has stopped holding fair elections (by barring many opposition candidates from even running), and knows that the more radical factions are the ones, in the end, that can be relied on to use force against rebellious Iranians.
If the regime ever falls, this will be the seam exploited by Western support for internal Iranian groups that will cooperate long enough to confront the powerful but minority cabal.