[AgentZero]

When I'm playing as an evil empire, which is most of the time, one of my favourite tricks is to ally with two (AI) opponents who are sharing a system with each other. I then park a small fleet over Empire A's planets, and declare war on Empire B. When Empire B's ships attack mine, I destroy them, but first capture the planet belonging to Empire A (my ally). Obviously this doesn't work against human opponents, but you can sometimes get quite a few planets this way.
I once had a game where I had fleets parked over 10 huge planets across 3 systems that belonged to my 'ally'. When I declared war on the other empire, he attacked all 10 fleets in the same turn (with 2 destroyers against 15 DNs. Go figure). I went from being ranked 6th in that game right up to 1st in one turn. Pure evil sweetness.