A method that has always succeeded for me (and I've been trying it a looonnngg time) is to work into it gently. Be patient. Give her a foot massage or two without tickling. Help her see that having her feet touched can feel nice. Use some quality lotion. Talk to her about other things while you're touching her feet (and don't forget to give compliments about them!). Then you can tickle lightly a little bit the next couple of times. Or, if you're both in a silly mood, you might be able to trap her feet playfully and tickle them for a few seconds.
Just don't let it be so deadly serious. If you go about it gradually, you'll be tickling her feet in a few weeks. I've even negotiated plans with women. If she really likes you, she might tell you that you can tickle her feet "once a week" or even "once a month." By then, as I've learned, an hour or so is a real possibility. Then you're in business.
Be careful of one thing: If you really like her, don't reduce your relationship to just tickling her feet. Tickling should enhance your time with her, not be the end result. Good luck! I've done well with approaches like this over the years, and I hope you can, too.
