I had tried, and tried when younger to tickle myself, to no avail, untill one day I discovered a way. Probably not the healthiest thing in the world, but, I cross my legs in the fashion that makes your foot fall asleep. When my foot goes totally numb, I uncross my legs, and wait untill the "Pins and Needles" effect starts, then I bounce my foot up and down on the floor. This tickles me intensely, and I have laughed, and even had to stop, sometimes, because I couldn't take anymore. As I said, probably not the healthiest thing to do, but I don't do it often.