Hmm.. I am not really sure how to increase your sensitivity. however anticipation of the tickling seems to work with some people. some think it's worse than the real thing but that probably wasn't helpful to you. LOL I tried.
PS: Here's a thought: I have a friend who is not ticklish unless it's her dad that is tickling her indicated that part of it could be due to who is tickling you. GO FIGURE!