Introduction: How to True a (really Wonked) Wheel
I got into a bike crash while racing last weekend. I Pringled my front wheel and severely wonked my back wheel (more than could be corrected just by a bit of spoke tweaking). I had to buy a new front wheel (maybe I'll try fixing it at some point) but this is how I fixed the rear.
Step 1: Take It Off
Step 2: Shape the Rim in a Vise
Step 3: True the Wheel
Put the wheel back in the fork. Truing stands are overrated - just use the brake shoes as indicators.
Moving around the wheel, gradually tighten up the spoke nuts. Revolve the wheel to see where it bulges or bows. Tighten up the opposite nuts to bring the rim back into line.
Truing takes some patience - invariably, tweaking one nut will cause unexpected warping elsewhere. Just keep at it and you'll get a sense for how the system works. If you feel you aren't making any progress, loosen everything and start from scratch again.