For the past 30 years I have been caring for an 1886 Casavant pipe
organ in Tokyo, and had a lot of stripped screw holes.  I tried
leather and steel wool and, yes, toothpicks also.  I have had the
best luck with the clipped-off ends of plastic cable ties, the ones
where you thread the tip through a hole at the end of the tie and
pull up tight.  They never rot or rust or disintegrate and the
screws can be easily removed if necessary.

Whatever you do, _don't_ fill the hole with sawdust and Superglue
(as someone has suggested) until you are absolutely sure that the
glue has hardened.  If you insert the screw while the glue is still
unsolidified, you have had it -- you'll never get it out again!

Good luck!
Vic Searle in Tokyo