There some tips in
this thread on 2-up riding that's linked from the sticky at the top of this area. The key points are:
- Tell them they can only get on the bike and off the bike when you have given them the clear OK to do so (so you are ready)
- They should not be afraid to hold on tightly, but try and find someplace that's comfortable for you both. Sometimes it's jacket, sometimes it's hips, but find something that works.
- As a rider, be very gentle on acceleration, braking, and turning. No matter what you do on a bike, a new passenger is going to think you are going 3 times as fast and 3 times as aggressive as you might think. They will be most impressed with a safe, comfortable ride before you work up to anything close to "let's show them what the bike can do".