How can you get people to tickle you?

tell them that you are ticklish and that you hate people tickling you

trust me i get it all the time

Next time you are alone with them, put a movie on and sit at the other end of the couch to him/her. Put your feet in their lap and ask for a foot rub. React in a ticklish way every so often, but do not remove your feet, just chuckle or twitch a bit.

If s/he apologises immediately say "That's OK. I actually really liked that."

Job done. And you haven't had to bring it up "out of the blue".