[Q]

I play against AI empires and I usually play a friendly empire style: That means I accept almost all treaty propositions and keep them, try to make fair trades (I do trade colonization technology because I think the AI profits as much as I do) and try to capture colonies and not to destroy them.
I heavily invest in my infrastructure and colonize every planet I can and as soon as I have the technology use atmosphere converters. In the late game I like to build ring- and sphereworlds until the game get's so complex (I usually play in a quadrant with 250 systems) that my computer takes more than a hour for processing one turn.