As already said really, try to relax which is not easy to start with and get your other half to go steady (I'm sure they will anyway) .
Watch out for accelerating and braking catching you out and try not to bang helmets! I get pillions to use a push pull method to reduce this .
Most of all enjoy it , there is nothing like being on a bike when it's cranked over!
Ps: I prefer the front to the pillion seat any day