How about breaks? Tickle a little, take a few seconds break, tickle some more...let her FEEL her powerlessness! also, tease her relentlessly! Run a finger over her stomach, saying 'think of how much i could really tickle you..' Wiggle your fingers in front of her. wave a feather in front of her bare feet. Like i said, tease her relentlessly. leave the full scale tickling till the end. Or do some full scale tickling, anther short break, more teasing, more ticklng...building up to yet another full scale tickle attack. leave her guessing. she'll never know exactly whats coming except at SOME point shell get the sh*t tickled out of her! The uncertainty and the helplessness will drive her crazy!
She may start begging also, which I personally love.
tickling is an art!