I've used (quite painstakingly) a 5/32 Nut Driver that I got in a set from the local Lowes. After a couple times of using it though, the screws will be rounded enough that it is going to really to be hard to get them out again. To have it work, you must be right on the screw (not at an angle on it) with the bit, and very straight and slow in removing with lots of pressure. It might be much easier for a DS where the screws aren't as deep into the case.
a $7.00 tri-wing screwdriver/adaptor is surely worth it it my experience.
Some folks have had luck with getting the hard plastic bic pens, heating the ends, pressing the end onto one of the screws, letting it set for a moment and pulling it out and cooling it. I've had limitted luck and once got the pen melted inside the long screw holes of the gamecube, not a good deal at all.
Search for the Mod That Cube site, it has lots of details to the pen trick.