Most of the time, if you get someone into a good tickling fit, they will naturally become more ticklish. But some things that work for me (or, more accurately, *ON* me):
1. Blindfold the victim.
2. Don't tickle one spot for very long... keep moving around. Vary the technique as well.
3. Taunt the victim... things like "does this tickle?" makes the victim more self-conscious and usually more ticklish.
Some people say that things like lotion will work, but I find that's only true if you're doing a vigorous fingernail tickle on the soles of the feet, which doesn't work on everyone.