Several people have suggested using a safeword for this. Something like: "I'm going to tickle you, even if you say 'no stop', but if you say 'potato' I'll stop

Several people have suggested using a safeword for this. Something like: "I'm going to tickle you, even if you say 'no stop', but if you say 'potato' I'll stop right away". I think this is something my kids would have done well with, though now they're just used to my taking their explicit "no"s literally.