Over the years I have used the tooth picks, match sticks with glue, small dowels with glue as Bill described but have evolved to using small triangular pieces of wood that I cut with a knife from the corners of wood scraps. It produces a taper triangular piece that bites into the over sized hole once you hammer it flush to the surface, will not spin when a screw is torqued into it and needs no glue. For small screws I use pine for larger screws requiring more torque I use oak and drill a pilot hole first.