Meh, I've done it a bunch. I used to commute to college in Chicago with my roommate/best friend on the back of his Guzzi. I felt a little guilty at stoplights because he's a twig and I'm...not, and he insisted (rightfully) that I keep my feet on the pegs while stopped.
Riding pillion is a skill (as is riding monkey in a sidehack). My girlfriend has had that drilled into her since day one, and she's great at it. She also rides, and the benefit of that coincidence cannot be overstated. Riders make the best pillions.
Otherwise, it's really just about trust. After a couple rides, you get to know what the pilot's going to do at the same point they know.
Anyway, as any of us can attest, the worst pillions are the ones that are terrified and stiff as a board. As long as they're paying attention to what's going on, the only thing the passenger really needs to do is look over the shoulder to the inside of the corner, and try to not bang their helmet on the pilot's.
Otherwise, relax and enjoy the ride. Riding bitch still beats sitting in traffic.