Take care of the feet, with cream and lotion. Clean them, treat them fine, etcetera.
When the tickling happens, use lotion, or warm the feet with a hair dryer, as warm feet are more sensitive.
And try different tickling techniques, start slowly and then go more intense.
Also, the main trigger to tickling is the surprise. Try to be blindfolded, or in such a position that you can not predict what will happen next. This will make you very ticklish, as the tickling is a natural reaction to alert you about strange things touching you... In this way, the tickling will be really from strange unpredictable things.