Well, it's always best to load up your first few planets with nothing but seed producing trees. The first few planets you get will inevitable end up being the core of your empire, and won't need that many defensive trees. Also, it's really important to find a planet with a really high strength rating and conquer it quickly. Once you have the high strength planet, it's best to wait until you have enough to send a half-half force to leave some on the mother-asteriods. From there it's more or less just a slow expansion until you have everything.
