I Usually build my 'Core Empire' (approx. 9 systems)first and defend all chokepoint w/ mines, sats or bases, allowing no other players to enter my space. Not easy sometimes.
When fortified I either research the other colony techs (or conquer them) and colonize all remaining planets in my Core Empire (I usually use Gas Giants but the many moons w/ spaceyards sounds good too
), or I build Stealth colony ships and build up some colonies 'behind enemy lines'. This makes it really fun to crush the enemy from 2 sides. Latter is really easy w/ Devnull Mod.
While tearing the enemy apart the 'Colony Empire' slowly merges w/ the 'Core Empire'.
Might not be very effective and take up alot of ressources but it challenges some more (oops empire C just killed my colonies...). I also role-play alot, that's the most important reason.
After all, it might not be the best tactic versus AI but it rocks versus humans (well, at least one time
).