Stop colonizing before the AI stops colonizing.
I'm actually very curious how you all accomplish this (after the colonization phase, of course. I understand the basic idea). I've tried this in a few games, both intentionally and inadvertantly and I have found that while I've had an early advantage, I can't hold it long enough to make a large enough impact, and generally end up losing via attrition. The only way I seem to hold the middle-end game is to set the races against eachother.
There are several ways, but they all involve moving against the AI to some degree while it's looking elsewhere.
For me, I usually move straight into social build up. This leaves me with a core of several planets capable of producing ships in a reasonable time (Especially with the Artifical Slave Center). I do NOT build up every planetary square. This core also lets me out tech at least some of my neighbors with the all factory strategy and focus. This becomes an advantage that the AI never recovers from...I flip over to military techs and a military build up with the remaining 75% social spending.
Depending on the map type, sometimes I just move straight to troop transports and start eating planets as they colonize them within my border and leave 2 or 3 Swiffers (named after the throw away floor cleaning brooms) to sweep any transports they send my way out of the sky while letting the colony ships continue through (and pick up their tech for free).
Truthfully, by the time the first real ships are coming on line they are almost indestructible. By the time the first ships are at the border, the second set is coming out and they are indestructible for a while...
And definitely keep them fighting each other!
