I went out and bought the Park tools for this and although they are expensive, I've cold set and realigned enough frames that I think it was worth it. You can certainly make your own without too much trouble, though.

Before you get all bendy on the frame, do the math.

1. Determine how far off you are from the correct spacing
2. Determine how far to move each side
3. Cold set it

1 is pretty easy. If the spacing should be 120 and it's 123 you are off by 3 mm.
2 Requires string or a gauge. If the dropouts centered 5 mm to the left of center then you need to move the left side in 4mm and the right side out 1mm.

So rather than just going from 123 to 120 and then trying to center it, which will usually involve more bending, just figure it out in advance and bend it once.

Hex wrenches make great measuring devices.

I strap a 2x4 to the rear triangle as a pivot to keep from putting too much stress on the brake and chainstay cross pieces.

And when you are done, don't forget to realign the dropouts!