Don't shift your butt around on the seat too much. Get settled **before** he takes off! Wait until you get to a stoplight or stopped traffic if you have to wiggle around... I know that seat's tiny, but wait until you're not moving.
Keep your feet on the pegs and don't distract him.
If you're on a sportbike, you're going to be leaned over against his back (not so much on a cruiser or standard). He's already supporting his entire weight against his wrists, so remember to support your own weight, but snuggle tight (this is the best part of having a girl on the back!). This lets him know you're back there and are okay, and he'll be able to tell if you're tensed up over something or are relaxed; it's also sexy as hell! There's nothing like having a woman pressed against your back like that! Trust me, he LOVES taking you on that bike.
When he stops, your weight is going to shift forward onto his back, though. Let it happen - he's actually expecting it and will be braced for it. When my wife started riding with me (before she was my wife), she used to let go of me completely whenever we stopped, and shoot her hands forward to try to brace herself with her hands against the tank! That completely screwed up my sense of the bike's balance, because suddenly her weight shifted and I had lost my "feel" of her back there. Umm... don't do that.
Lean the way he leans (which, if he's doing it properly, isn't really leaning at all - it's the bike that's at an angle, and you have to "lean" away from **it** to stay sitting straight up).
Stay relaxed, and ask him to tell you if he needs you to do anything different.
Enjoy yourselves!!!