Soles Scream - If you are planning to tickle underarms and you are not starting with the biceps, if not the palms and wrists themselves, you are doing something wrong.
Door 44 (or one of their other companies) - Tickling is often about the spots where body parts meet... Toes and soles, legs and groin, or in this case, the small of the back and the top of the butt.