I am by no means an expert but here are some things you can try...
Try catching her by surprise so she doesn't see it coming and thus can't fight it. Or not tickling her right away but waiting until she relaxes and thinks the tickles are not coming, she can't keep blocking the feeling forever she is going to have to loosen up just wait for it (You will definately notice when she does). Dont let her know you are waiting for her to do just that. Usually it is when she starts to say anything, thats when you strike before she is done talking (try to catch her in mid-sentence, or mid-breath).
Don't expect her to laugh or react immediately once you start tickling. Expect that she will resist as soon as you start tickling so you can stop. If she doesn't laugh yet keep trying she may hold back the laughs but if you keep going she will burst like a dam. Everybody has their limits. Also she may try to make excuses for you to stop don't fall for it, keep going (listen for any strange sounds in her voice while she talks).
Try some tickle taunting and teasing (coochees coos, tickle tickle, anything). Yes it may sound stupid but it helps break down the mental defenses, as she focuses on your words not the sensations. Also try saying things like "I am going to tickle your feet forever or for a long time". Even if its not true just to psyche her out and continuing breaking her willpower she may be using not to laugh.
Try talking to her as you tickle her, about anything(everday life/normal conversation/thought provoking topics) because if she holding out talking to her will make it harder to do that. The idea is to try to make her concentrate about anything other then the tickly feeling on her feet. Get her to talk to you more though, so you can listen for anything(ha ha's, giggles, snorts, he he's, pauses, sharp intake of air)that might slip out that she may otherwise try to hide if she was not talking. Thus letting you know that your technique is getting to her.
Try to tickle her for an unknown amount of time (to her) because if it tickles and she doesn't know when it is going to stop it is going to be harder for her to know exactly how long to hold out for. If you do have to set a time limit, lie and say instead of the time limit agreed upon that you will tickle her for an outlandish amount of time (hours, days, etc..) say this once you start tickling not before to get her to focus on your words not what she feels. Something about being told that your going to be tickled nonstop that just makes the floodgates open. 🙂
You can also try a blindfold so she can't see it coming and try to block it out.
Try to mess with her mentally, by talking about how ticklish she is, or how feet just coming out of shoes and socks or just having your feet pedicured make them more ticklish (it doesn't have to be true 🙄 ) make something up. The idea is to make her think it is true make her worry, thinking about how ticklish a spot might be or by forcing her to anticipate the feeling of that spot being tickled makes the laughs come right out.
Tickle her in a spot you already know that is ticklish, just to make her laugh and then tickle her feet, because she can't fight the laughter if she is already laughing.
Tickle her feet with anything(feathers, forks, ice,toothbrushes, fingers, etc.)Anything!!! Different things have different sensations on her feet that all end up tickling and if she is expecting one thing try another (with each foot being tickled with something different if it comes to that).Remember tickle everywhere(between/under the toes) on her feet with everything you can think of she can't be prepared for everything and block out all those feelings on her toes and soles. Don't be predictable mix it up, strike at random times at random places with all kinds of "tools". If anything it will help her feel the tickling feeling that you want her to feel that eventually makes her laugh.
Be as silly as you possibly can before and during the tickling of her feet, get her to have fun. Get her to relax (so you can strike), Get her to talk ( so you can strike), Get her to laugh from your silliness (so you can strike), Get her mind off the tickling (so you can strike), get her to anticipate the tickles (so you can strike). You know the idea. :tounge:
Remember the basic idea is to break down and get the past any willpower that allows her to block out the tickling sensations on her feet so you can make her laugh by any means necessary. Anything you do is fair game, you don't want to lose do you? :sharp_tee
Also listen to the people here, all the advice helps believe me 😀
Of course there might also be the slim chance that she is actually not ticklish on her feet, hey I know it happens not as often as people lying about not being ticklish there or being ticklish in general. But remember there are some non-ticklish people out there they are few and far between, but they are out there but most if not all of them are either: liars or just have not had there tickle spots found and its up to us them find out :tounge: