I pump a few brief flashes from a small LED flashlight, held over my head. Sometimes I aim at the white wall nearby. The virtue of this approach - using a separate light source - is that the exposure the paper receives is not dependent on the density of the negative ... and it shouldn't, since the optimum amount of flashing depends on the paper, with only the highlight density of the negative as a slight contributing factor.
The downside of this approach is that it's hard to time accurately.
On other occasions, I've used the backlit screen of a PDA (a cell phone would work too) to flash paper when it's in the developer. I may start using that method more, since it's easier to time a weak light source like a PDA.