I assemble a team of favorites that all work together. For White (as I usually do), I took all of my favorites and picked a Grass-type, a Fire-type, a Water-type, and a Flying-type out of them and made sure I had a good mix of Physical and Special attackers, and not just a bunch of sweepers, but some Pokemon that could be tanks. I added in two more Pokemon with that same method in mind.
Black has been more of the "leftover" team, though I'm still trying to use certain Pokemon with certain others (so as to avoid a team full of Physical attackers, etc.). I decided not to use Tirtouga with my Emboar and Leavanny because I knew I needed Frillish's special attack abilities on my team, for example.
So, largely similar to those above--I use my favorites. I just make sure they make a cohesive team before running a game with them.