First off, this is a pre-patch exploit only. Avoid updating the game for a while if you plan on using this cheat.
Find the potion vendor by Town Square in Bowerstone Market. Go around the right hand side of the buildings by the potion shop, and down an alley you’ll find Monstor Manor. If you sleep in Monstor Manor you’ll get +1 Physique, which you can sell repeatedly for experience that can be used for any skill. The more stars you have in Physique, the more experience you will gain.