I just happened to be leaning against a wall a few months ago, with my arm extended and my palm on the wall, and even though my armpits are my most ticklish area I didn't even think about how great a position I was in until one of my female friends came up behind me and wriggled her fingers into my armpit. Of course I almost fell face first into the wall, which was probably why she did it, but just about everytime I'm around her now I make sure to lean on something.
So not only did I get a nice surprise but I also ended up with a pretty good way to get tickled cause let's face it, people falling is funny! Since pushing someone to make them fall is a mean thing to do people really prefer tickling people to make them fall, now if only they'd choose tickling as a way to do a lot of things.