Depends on how many and what kind of enemies are in the target world. Usually the defenders are scattered around, so if you scout and plan ahead of time, you can usually take out some of them quickly before the others make their way from the other side of the gravity well.
A cheap way to claim a terran planet is to kill the siege frigates protecting it quickly (SF's are very weak, so this isn't hard), and then use a colonization cap to take over the planet. Now, quickly build some defense platforms with your new construction vessels and use them to kill off the remaining defenders. You can also begin planet upgrades and such at this point. This strategy works well for Vasari and Advent. For TEC, a good approach is to just send a Marza in there gung-ho and let it clear out the gravity well. Good experience, and you don't lose any units. Wouldn't try this on a small map, though, since your Marza may be needed to stop an enemy rush and having your shields already depleted wouldn't bode well.
