When my wife first started riding with me, I told her that once her feet were on the pegs, they need to stay there until we're in a parking spot and I'm turning the bike off.
Lean with the bike and look over my inside shoulder when leaning or turning. If I'm turning left her head should be either directly behind mine or looking over my left shoulder. That keeps the weight of the bike moving toward your turn. If she leans way to the outside or shifts her weight to the outside you could miss your turn... or worse.
My wife likes a sissy bar or luggage case behind her. She is much more comfortable if something is pushing back into her... and I'm not horking around with her on the back. She just likes being able to feel that there's something behind her to help hold her on the bike.
Tell her she can move around quite a bit while you're going faster than 30 mph. She can readjust seating position much easier (for you) while moving. Below 30 mph, she should try to be as still as possible on the back seat.
Once you get into the groove of things, that 30 mph rule can be dropped down to about 5-10 mph. My wife is a master at this now. Once we're rolling I've seen her taking pictures, pointing out the sights, watching traffic, warning me of impending dangers (like a bouncing ball coming down a driveway with a kid chasing it
), taking a drink, offering me one, etc.
Most of all, keep the rides short in the beginning. You can always go for 5 separate 10 mile rides and then call it quits, but if she's done after 30 miles and you're 25 miles from home... well, that last leg of the ride won't be too much fun for you.