I've been around the block a few times, and I can tell you that the key to getting what you want is confidence.
Now, confidence is not the same thing as arrogance. I'm not telling you to act like Barney Stinson. Just act like you know what you're doing. Give your actions a sense of purpose. If you're going to tickle a girl, do it without hesitation. Put on a confident smile and make it seem like what you're doing is totally normal. Don't hesitate, don't stutter, don't seize up, and generally, don't be awkward.
Keeping a confident demeanor does take practice, though. Not everyone is born with natural confidence. You need to work on it. As someone said earlier, "damage your internal giver of damns." Go for it the first time without worrying about the consequences. You should only be concerned if you get a bad reaction, which isn't that big a deal. Just leave her alone and find someone else to try it with.
Above all, try not to obsess over your tickling fetish. Yeah, yeah, sexual gratification is nice, but don't let it consume you. The more you think and worry about it, the more stressed out you'll be. Just relax, think of other things every now and again, and when you see a good opportunity to tickle a woman, take it. Simple as that.
Hope this helps.