It occurred to me a bit later that this technique isn't 100%. Anyone who's been traumatized via their bottom (abusive parent constantly using belts on his behind, or maybe an unsavory prison shower experience, or whatever) might have issues "thinking about his butt in the chair".
In such a case, perhaps just taking a walk and stopping to smell the roses (unless they smell like the perfume your abusive stepmother always wore) or comparing the leaves of trees (unless you were once trapped for several hours in a freak leaf avalanche) might be useful.