Of course it depends upon the person, but I'd recommend first and foremost to think of the feet, not as a tickle spot, but as a region of tickle spots. Covering more space at a time is not necessarily the best long-term strategy. Also, doing one foot at a time will give you a whole new world to switch to if the first foot gets desensitized enough. But for the actual tickling, try focusing on a limited tender area and lightly scratch around there for a while. Occasionally move to a different spot. Do each toe individually. Spend time between each toe. Try the tops of the feet, the heel, the side, the sole, etc. Before switching to the other foot, it's a good time to do a broader scrubbing with a brush or similar implement, but don't go too hard. Not only can that be approaching the pain threshold for some, but it can lead to desensitization over time more easily. Switch to the other foot and spend a lot of time there doing what you did to the first one. The sensitivity of the untouched foot should be pretty high because the brain has gotten used to all the tickling being on the one foot. Eventually do the scrubbing. After a while, switch back to the other foot again and wash, rinse and repeat.
