There is a compound sold in hardware stores that can be used on the joints of the legs, where they insert into the seat. It causes the wood to swell and tighten. I'll be darned if I can remember the name of it, though, and I can't look it up from where I am right now. However, it will be in the same section as furniture finishes and treatments. The Homer Formby line of products is a good place to start.