My own personal method is to strap a bent spoke to the stay/fork. I add a round at one end so as not to be poking at my rim with a sharp bit of metal and then an inch or two further down I'll add an L-bend. Now strap (velcro ankle strap works pretty nicely) the long part of the L to the stay and rest the lateral "arm" (short part of the L where the round is) against the rim wall.

You can rotate the spoke shaft to adjust the distance between the feeler and the rim. Strap another to the other side and position it over the top of the rim and you can do lateral and radial truing simultaneously (assuming the rim doesn't have any actual blips). This is amazingly useful as you can simultaneously notice something along the lines of, the rim is is a little flat, pulling left, and has pretty high tension to the left flange. Being able to see all those things at once means that you can immediately recognize that all you need to do is let a little tension out of the left spoke to correct the lateral and radial truing in one simple move.

Using this method, I've built wheels as true as anything I could buy at my LBS.