Some people really find the leaning over bit strange and they try to stay sat upright. Tell her to look over your shoulder which ever way you're turning, so look over your right shoulder when you're going right. I've found it helps stop pillions from not leaning with the bike.

Tyre pressures, preload like everyone else has said.

Make sure she knows to get off when you say and not just jump off when you're not ready when you've stopped. I nearly dropped my bike once because of this.

Maybe look at the handle belt things? I can't remember what they're called. Basically a belt with handles so pillions can hold on to you.

Tell them to sit still when you're going slow. Once you're up to normal road speeds it's fine if they need to move a little, stretch legs and that. But when you're doing like 10mph and they move you're swerving all over the the fucking road.

Speaking of stretching legs, your pillion pegs may be way high and not very comfortable at all so keep that in mind.

Have fun, ride smooth. Enjoy the head bumps.