Trade your influence points for all the AI Civ's cash. Unless you have more than 50% of total influence to make sure your vote wins the UP, otherwise IP's are pretty worthless. On my suicidal game, I raked in over 30000 BC in 1 turn from trading IP, of course, I don't do that too often, as it's quite time consuming.
Buying cash is always nice, but if you are truly insane you can buy almost anything with endless repititions of that theme. Buy all their cash, then use that cash to buy ships, tech, planets, treaties, etc. After each purchase, you will need to stop and rebuy all their cash, so it's a slow, carpal-tunnel-inducing process - but it works. If you start it early enough, you can keep even suicidal AIs in check, to the point where they never expand enough to be a threat.
My current game, I bought out two empires on the first day I met them, to the point where they only had their three most populated palnets left. Given the layout of the planets, one flipped in short order, the other 5 could at any time (trying to avoid that, they both have tech I want them to research first). The only thing you can't buy this way is mining bases and an empire's last three planets.
The important thing is to catch the planets right after colonization (or invasion, if you let them get that far). The value the AI puts on a planet is based almost entirely on population; if you can get the Korath to start sporing everyone in sight, you can buy those planets for a few hundred IP, or sometimes under 100 bc.
NOTE: attempting the preceding strategy is a MASSIVE financial strain. Even the best economy will be sucked dry by this for the first two years; your only hope is to buy bc off the AIs and minors faster than you can pour them down the sinkhole that is your colony upkeep (I was losing almost 3000 bc a turn at the beginning of the second year). OTOH, last time I tried this I ended the colony rush owning all but about 30 planets.