[Wanderer]

I tend to find Puppet Political Parties don't work as well unless the enemy planets are full of angry people. Try several turns of massed Anarchy Groups projects then start hitting them with PPPs. The Anarchy Groups also reduce the enemy's production which is always a good thing. Unless your opponents have Urban Pacification Centers, hundreds of troops etc... the effects can be quite long-Lasting.

Another thing is... make sure the PPP isn't the first thing on your project list (best of all make it the Last project on your list that'll complete in one turn) - I suspect it's more likely to succeed as the first projects use up the enemy's Counter-Intelligence points so they can't resist the later projects. Might not be the case, but it seems to work most of the time.

Oh, and don't be disheartened if your first successful PPPs merely cause enemy planets to rebel and declare independence. You'll eventually get planets defecting directly to you. The single planet empires will usually surrender to you (along with some tech) if you enter the system and challenge them.