Like said before, make sure they know what to do.
Two of the biggest things I learned when I first rode passengers is that they dont know how to brace themselves when slowing down and also braking is more difficult.
When I ride a passenger I tell them before we leave that they can brace themselves when slowing by putting their hands around me and onto the tank, leaning on me, (and for the more attractive women) putting their hands on my thighs and holding on
If they dont know how to brace themselves the first time you let of the throttle their torso is going to slam into your back.
Also definitely leave more room for breaking. My maiden voyage with a passenger almost ended badly when I was coming to a stop at an intersection with my front wheel turned very slightly and i applied to much front brake. It caused my handle bars to turn completely sideways, stopped us abruptly, and I almost dropped the bike. And with a passenger on it your holding up the weight of the bike and the weight of a passenger. Somehow I managed to go hulk status and lift it back up.
With that said, I now do not use the front brake at all with a passenger once I am under 10 mph.
Also, another good thing to do is try to shift a little sooner so your helmets arnt smacking together through each gear. Also give them a type of signal that they can give you if that for any reason they need you to pull over, perhaps for an emergency.