Great Caesar's Ghost!!!
Taunting is required!!!
When I was a youngster, my aunt used to tickle us to death. She would say things like, "My, my, my. Are we ticklish? Only little girls are supposed to be ticklish." She would taunt me with things like that while she was making me turn blue.
With my wife, I ask her things like should we get this rib for a while. And, "Isn't this fun?" When I have her in silent laughter, I will ask her if she likes this in the same sentence that I ask if she wants me to stop. All she can do is shake her head and whichever way she shakes it's guaranteed to make me tickle the more.
Once years ago I had my mother-in-law pinned to the floor with her arms over her head under my legs. I gave her such a tickling. When I stopped before I let her up, while her arms were still pinned down she said, "I'm going to kill you!" Well! Before I finished the rest of her tickling, I made her say I was BY FAR her favorite son-in-law. I made her apologize for things she never even did. I made this fifty year old woman tell me she would from now on be a good girl.
You have to taunt for the full affect. Half the fun of tickling is teasing the bejeebers out of your victim. I like to think it's partly why, to this day, my wife will still try to run from her ticklings. A promise I never get tired of chasing her even though we are both too old for this sort of activity...well I'm not too old.
Hiram