Let's Make a Deal
.
Tell them they have a choice of where they will be tickled next before you let them go. They must choose 2 of the following 6 areas of their body: feet, knees, sides, bellybutton, ribs, armpits. After they have chosen the two two, proceed to ignore those and tickle the other four vigorously. Deal must be delivered solemnly and with an absolutely straight face. Better if victim is not too bright. (This obviously gets played at the end of a session.)
.