Hang the bike on the wall. Leave it there.

At the most basic level, you can true wheels pretty simply using just a spoke wrench and looking at where the rim moves side-to-side relative to the brake pads as the wheel spins. Unless you know the wheel is seriously out of whack, the best way to true is to loosen up one spoke on the side you want to move away from, and tighten the adjacent spoke on the side you want to move the rim to. Think of "conservation of spoke tension"- if you loosen one spoke a quarter turn, tighten the next spoke a quarter turn. You don't want to turn spoke nuts more than that at a time on a wheel that's basically true and just needs to be touched up. In fact, if you're trying to move a significant section of the rim over several spokes, you may want to work in 1/8 of a turn at a time.

If this doesn't make sense, try taking it to an LBS and have a mechanic illustrate what I'm talking about. He may explain it better than I can anyway.