I am sure others do this, but here is the kicker. Trade one treaty with the two strongest AI for one of theirs. Since its so 'valuable' they will give all there techs and up to HALF of their planets. But just as important you ask for all of their scout ships, as well as their miner if you can get it. I go to my shipyard and design a small colony ship with only the colony module and life support. Upgradeing a scout to a colony costs about 175. And presto you have a fleet of colony ships all over the map. Nothing more satisfying then to 'ninja steal' a planet all the way across the map from the enemy.
Now for the Dirty tricks. Since humans also have a influence bonus, take a planet with low population (send nearly empty colony ships there and 'fill up'). Pick a planet deep inside your influence. And trade it to the AI. Soon afterward it 'converts' back to you and you do it again. The more population on the planet the higher the 'value'. So building a ring of influence starbases around 'Planet Swindle' helps get the planet back faster and lets you keep the poplation up for higher percieved value.
Planet Swindle can be done in more ways than one, with Super Diplomats at least.
Whenever an AI race acquires a new planet, whether from colonization, invasion, spore, culture or a surrender, they are always willing to sell that planet on the same turn or a few turns afterwards for dirt-cheap prices ranging from ~1500-2500 bc depending on diplomacy rating, and completely irrespective of the new planet's population or infrastructure. After that critical buy-time though, the AI inflates the price to "you'll have to compensate us" levels scaling with population/infrastructure, or simply refuses to sell at all.
In a recent game as the Terrans, I had bribed the Altarians, Drengin and Arceans to attack the Drath as part of my usual war-inciting routine. Of course, the Drath got their already-weak military crushed, and so they surrendered to the Yor. Just after I got the surrender message from Draken, I got N-1 on the Universal Translator and bought up all the key pre-built, pre-populated former-Drath worlds for ~1500 bc apiece, with their "homeworld" of Dratha going for ~1800 bc. Since the purchased planets all had 5-15 billion population on them, the minor worlds soon flipped for free.
In fact, as a Super Diplomat, it's rather pointless to invoke a war and go running around with troop transports of your own to pick up planets. Just let the AI invade and take over worlds, and keep buying them up. The cost of planets is less than the combined cost of invasion technologies + transport ships + troops + invasion tactics, and there is far less micromanagement and hassle involved. And you have no need to risk a war yourself and get tangled up with a Secret Meeting event or the victim's Allies.
The same thing can be done for the colony rush, and I've so far done it thrice as the Terrans. I colonized planets in my own star cluster and made them economy worlds with morale at 100%, unless they have a bonus tile in which case they become production worlds. Then I watched as the AI colonized by tracking the change of cultural influence, and kept buying up planets for low price while I myself researched economic/diplomacy/high-priced techs. Trash technologies like Sensors or General Life Support, or Space Mining would fly off the shelves with AIs paying through their noses in the form of planets and money and/or scouts & miners. If I ran short of money/junk-tech, I'd contact a Minor Race and get some more money or a useless Armor Theory or something.
Eventually I'd end up with planets all over the map and a sum total of influence high enough to block any flip threats. Only once did I ever get caught in a crappy deal - when the Yor sold me a Class 2 Barren World. Too bad, that after all Terraforming had been done, it turned out to be a Class 23... and later proved central to a cultural assault which brought down the Yor themselves.
