First time on back with me? I'd tell her this: "Relax and just stay with the bike (your back in the same orientation to it as when you are sitting behind me not moving). Whatever you do, don't lean to the outside of the turn. Don't move around on the bike, and if you have to move, do it subtly, slowly and smoothly. This is especially important if we are doing something other than going in a straight line or if we're slowing down."
I absolutely agree on the backrest; makes it much easier for a pillion to relax AND to: "just stay with the bike."
Advanced pillion that I've ridden with a while? Different instructions, to an extent varied by my assessment of her innate feeling of carving through a turn, the way she reacts to me and the bike and whether I sense that she and I are on the same page about everything from seeing and reacting to potential threats to looking all the way through corners. I'm lucky to have a great pillion again. She is an expert equestrian, so her reaction to other controlling entities between her and the road is pretty close to perfect. She's used to having a horse's reaction to terrain as a baseline, so her reaction to me manipulating the bike on the road is never a distraction. She sees what I'm reacting to and why. And she's got some advanced abilities -- practiced panic stop reaction, the ability to let me move freely left and right on the seat in fast twisties, and a sense to follow my body in sweepers. A good pillion can minimize the effect of the extra weight.
If you've only logged 4,000 miles on motorcycles, though, first thing is to be conservative and cautious. A pillion is a noticeably greater weight, up high to make the bike more top heavy. If she's inexperienced, you need to be ready ahead of time for what it's going to be like in a panic stop or hard turn (e.g., a decreasing radius turn where the bike's suspension compresses) and be able to handle it -- ignore it and do what you have to do while other things are demanding your attention. She's likely to have all her weight on you (which will be borne by your hands on the grips) in a panic stop. (As you log more miles together, teach her to plant her palms on the tank with her arms around you in panic stops.) The extra weight (even if she remains neutral) dramatically changes your ability to drive the bike lower (lean it more) in a turn you entered too fast. Your suspension will bottom out more easily in hard corners, etc. So, most importantly: make sure you get very used to riding with a pillion before you get anywhere close to the edge of the envelope you've so far defined in your riding experience.
Good luck, be safe and have fun. The bike won't perform as well two up, but it's great sharing the fun, thrill and view of the world from a moto-bike with a wife or girlfriend. Well, hell -- having breasts against your back is NEVER a bad thing, either.