This thread is my jam! I'll tickle feet or anywhere else but upper body is definitely what gets me going.
From the top, if I'm intimate with the woman then whispering, nibbling and blowing on the ears and neck can get her giggling and purring like a kitten. You only need a soft touch here. I tickled one girl where all I had to do was tickle the back of her neck just at her hairline and she would instantly turn to giggling jelly... and then she'd start breathing hard.
Underarms... for a session, I love to tie her on her back with her arms overhead. When she's this vulnerable I just like to look her in the eye while lightly scribbling in her underarms, almost like my fingertips are brushes. I've actually had to stop before the safeword with one woman as she just couldn't take it at all. However, when she was untied, tickling her from behind and getting my fingers in her pits worked like a dream. Ticklees reflexively clamp their arms to their sides, trapping your fingers exactly where you want them.
You can be a bit more vigorous with the ribs but when I was starting out as a tickler I would sometimes prod too hard. This taught me to listen to a ticklee and her body so she can enjoy it too. It's fun to tickle on the couch and get the ribs too in a 'tickle hug' but when she's tied down, I can't deny that tickling her ribs near her breasts and make them giggle and jiggle really gets the blood flowing, whether she's topless or not.
If I have a fetish for any part of the female body it's the stomach and I was lucky enough to date a woman once who had a slim, athletic belly and a perfect oval of a belly button. She could have had her belly on women's fitness magazine covers. Needless to say I couldn't leave her alone. Getting her waist with my thumbs either side of her belly button was a good one. Light tickling with the pads of the fingertips around the belly button works well. I've never found 'poking' the belly button to work and sadly a lot of women I've tickled just weren't ticklish there. But... Miss sexy athletic belly most definitely was (in fact she was all over the most ticklish woman I've had the pleasure of tickling.) Nibbling gently around the rim, gently blowing on her skin, tongue tickling, raspberries or gently pinky finger tickling (if she's stretched taut) were the winning formula. I will never got bored of that.
Finally, the hips! I love to 'pickpocket' her from behind. Getting my fingers in her jeans pockets kind of traps them there and I haven't yet found a more effective 'tickling takedown' to turn a woman's knees to jelly and turn her into a giggling mess. At that point, she's getting straddled or carried to the couch or bed, whatever's closer. If she's tied down I LOVE to get the hips and belly button at the same time (insert evil moustache twirling villain laugh.)
Finally, I usually like to start sessions with my ticklee clothed so I can assert my dominance by undressing her as I go. Then again, my regular tickling partners have been women I've been dating or in a relationship with so that becomes part of foreplay.
Anyway, great thread!