I try to make it a point to stimulate my girlfriend elsewhere while I tickle her feet. I would love to get a pavlovian response going that having her feet tickled would make her associate it with sex.
Only the ones into tickling...not all women.
I can get off from having my feet tickled...my wife, not so much. She, in fact, cannot stand to have her feet tickled. I only get to rub/kiss/worship them...which is ok with me.