When I was an upper classman in high school I had a good friend over. I used to tickle her sometimes but she didn't know about my fetish. She told me that even though I tickle her all the time that she actually has a very strong will power and could probably go without laughing or reacting completely if I were to tickle her feet, so proceeded to challenge her statement.
I held one of her ankles up toward me and ran one of my fingers down the arch of her foot. She didn't budge. Usually this would be enough to make her at least flinch or pull her foot away a little. So studying her face and body language carefully, I decided to continue. I ran my finger up and down the bottom of her foot a couple more times and she still didn't react much at all, but still trying to assure me that it wasn't bothering her at all. After that I started picking up the intensity a little bit. I took her other foot as well and wiggled two fingers each up and down the bottom of her feet from her heels to around the balls of her foot. Now we were talking a little bit she was saying things like "see? I told you I can handle it" and was saying that she was good and wasn't going to let up. I could tell by the way she was smiling and talking to me that she was probably starting to lose it a little bit, but I couldn't be too sure. I stopped for a couple of seconds in between to switch positions and try something else. It has now been about 45 seconds to a minute and I still haven't gotten her to break yet. I was determined. I couldn't lose my pride as a tickler by watching her totally keep her cool while I'm tickling her soft, vulerable soles of her feet. I took one foot and wiggled 4 fingers all around her sole. Even getting under and around her toes, which is basically her weak spot. "Are you impressed yet?" She kept asking me.. I took her foot and wiggled 4 fingers right above her heel where the arch starts, and then wiggled the other 4 fingers on my other hand on the ball of her foot/under her toes. After about a second of this she made a strained face, was breathing a little quick, curled her toes and then let out a little whimper/crack of a giggle as she pulled her foot away from me.. 😀
After this happened I took both of her feet again and with both hands tickled all over the bottoms of her feet, and she tried to hold on again for another couple of seconds and then laughed and pulled her legs away desperately.