My Suggestion........
I would say have someone tickle you with small, thin, and soft objects (feathers, fur, long fingernails, etc.). Also, let someone tickle your feet while you have socks on. Then if that works, let him/her take your socks off and tickle your barefeet. A girl-friend of mine became ticklish on her feet this way, and she loved it. I loved it, too. It may not be possible for everybody, but it's worth a try. Things can go the other way around, too. I once had another girl-friend who lost her ticklishness as she got older. She got tickled a lot as a child. But as she got older, she was no longer ticklish. But at least she could enjoy a foot massage without squirming around so much. Some people can't stand having their feet touched when they're ticklish. But I prefer a woman's feet to be ticklish; the more ticklish, the better!