Park Tool has a decent discussion of how to true a wheel:
http://parktool.com/repair/readhowto.asp?id=81
Zinn and the Art of Road/Mountain Bike Maintenance also has a good discussion with good illustrations.
The Art of Wheel Building by Gerd Schraner has the best illustrations I've seen of the truing process and is spiral bound so you can easily reference them while working.
I had a new wheelset that went out of true bad enough that the brakes were rubbing within the first 6 miles I used it. Using the above resources, I got it back into true in a matter of minutes on my first try. A decent truing stand is handy to get things perfect but you *can* use the brake calipers for reference in a pinch.
The basic concept is to work with the 4 spokes around the bend in the rim. Loosen *slightly* (1/4 turn or so) the two inside spokes (at the apex of the bend) and tighten *slightly* the two outside spokes. Repeat as necessary until the wheel is true.