Even if you don't own a truing stand or a dishing tool you can still take a good first crack at truing a wheel so that the rims run straight.

I flipped the frame of my bike upside down, installed the wheel in the front fork, and then attached a ruler (metric, natch) to the fork with elastic bands.


(Click to enlarge)

Get the ruler as close to the rim as you can without touching it, then give the wheel a spin. The ruler acts as a fixed reference point against which you can evaluate warps in the rim.

Watch a video of the DIY truing stand in action:
http://www.flickr.com/photos/steveluscher/8338976590/ Using the markings on the ruler and the width of the rim, I was able to roughly calculate where the rim should sit. Wherever the spinning rim would "cross the line" I would tighten the group of spokes opposite that point. Sometimes, instead, I would loosen the group of spokes on the same side as that point, in an effort to maintain the overall A440 tuning of the spokes.

After the rough home-true, I went down to Our Community Bikes to finish the job on their more precise truing stands and dishing tools.