DON'T EVER BACK TO BLACK YOUR REAR SEAT EVER!
Made that mistake once, guy on the back slips around like a b*stard and nearly slider off the back when accelerating hard.
Tell your pillion to relax and invest in some love handles. Having their arms around you makes cornering very tricky and uncomfortable. Also, get an intercom, however much fun it is having someone to talk to, shouting your **** off down the motorway quickly detracts from it.