This been discussed over and over again, I think once this week even. A search would have yielded plenty.

But anyhow:
1) check that your axle isn't protruding past the outside face of the dropout. If it does, readjust, or add washer inside locknut. If you need to add a washer, do it on the non-drive side, or be prepared to readjust the shifting.
2) check that your q/r springs are correclty orientated, pointy end inwards. Or remove them.
3) check that the outside face of your locknuts are properly serrated. If they're worn smooth, replace. If replacing is somehow difficult, you can use a centering punch to punch dimples into the outside face, or a chisel to create some grooves and ridges.
4) If your q/r has external cam, replace with an internal cam, steel q/r. Stock Shimano ones do fine.