Riding on the back of a bike is a wonderful experience. The first time you do, it can be very liberating, but also very scary. You may not know what to expect or how to act, but there are a few things that you can make sure you do to help make the ride easier and more comfortable for everyone involved.
First, make sure that you are wearing the appropriate safety gear. Just because you are on the back of the bike does not mean you are exempt from wearing at the bare minimum jeans, a good jacket (preferably leather), and a helmet. You may think that you look exceptional in your tiny outfit with the motorcycle patches or a bikini, but if you're in a wreck, you could suffer from multiple lacerations, burns, and disfigurement. So be sure to keep your safety in mind.
Second, do not jump off of a bike or try to dismount, the second that the motorcycle comes to a stop. If the driver is not ready for you to get off and you try to dismount, it could cause them to lose their balance and lay the bike over. This is not a good scenario for anyone. So, make sure you have the all clear before you jump up off the bike.
Lastly, when you sit on the bike, make sure that you put your feet on the appropriate pegs and that you sit upright. It is important to strike a good balance. Do not be so stiff that you don't move through the turns and slopes with the driver, but remember to sit still for the most part.