"Toothpick," if you are in the US. The hardwood variety, not the cheapies.
I have done lots of woodworking and a bit of luthiering and I can't think of any better way to make this fix than what Stu suggested.
If the hole is really buggered you can put two slivers in. If you make the hole too tight, just run a small drill bit CAREFULLY back into the hole so the screw gets a new bite.
-Glenn