I'd say short of wiggling your bare feet in their face with a hopeful look on your face, there's not an awful lot you can do to invoke a tickling attack...
Maybe you could try talking about how ticklish you are, and the other person might try tickling you to test it out.
This is problematic if you're after some involuntary uncontrollable laughter... Perhaps another idea would be to start tickling someone else, and hoping that they tickle you back in response. I'd be wary of this though, they might give you a slap instead!
Science of tickling
In the late 19th century, Charles Darwin suggested that tickling is a mechanism for social bonding: Parents with their new-born children, for example, or as a means of flirting between potential suitors.
You can't tickle yourself! Funny, isn't it? But somewhere in your brain the reaction that tickling produces is pre-empted, so if you do it yourself, your body is prepared and your nerve endings aren't as sensitive.
Check out Cookie the penguin getting a little tickle!