First thing I did with my missus is warn her about the hot exhaust, tell her to keep her feet up at all times and keep her shoulders exactly in line with yours. If you lean, she leans, go with it, relax and don't fight the leaning around corners (like one of my mates does!!).
Get her to rest her chin on your shoulder to stop headbutting under braking, just keep her body against yours to prevent sliding .
oh and tell her she has to stay awake. My missus has a habit of falling asleep at motorway speeds!
Just build up the distances and you really don't need to go mental for her to have an exciting ride.
...and if she does slide down the seat, tell her to slide her arse back as one. My missus shuffles and 'walks' her arse back up the seat which results in a big wobble usually on approach to a junction.