-Kunoke- has a good idea, if you feel like you're bold enough. However, if you're like me, and you get that knot in your throat just thinking about it, first chill and take some breaths. Then simple offer to massage her feet. Trust me, not many women will turn that down.
Then you can slip in a tickle. If/When she remarks about it, then you can tell her. I usually preface it with "I'm so sorry, I couldn't help it. I just had to tickle your feet. I hope that's OK with you." - keep it sincere but not too sappy.
OR you can say instead, "Oh, are we ticklish?" and tickle some more. Then follow up with something along the lines "Oh, I just love tickling you." - here, keep it playful but not creepy.
Hope it helps. Anyone think of anything else?.