Tips for the Rider
Start slow. Engage with your pillion and show them all of the different parts of your motorcycle. Point out the exhaust (and how hot it can get so be careful!), your/their foot pegs, etc. Get your passenger to learn a little bit about your bike and thus making the trip easier once everyone knows how it works.
Communicate with your pillion! Discuss what to do if there is an emergency, what the route is, and what to expect (bumpy roads, pee breaks, etc.). Ask them if they are ready before you take off (even better if you have a set of Bluetooth communication devices).
Starting off, I would have your pillion get on/off the bike a few times (remember to get on from the left side). After you are comfortable with the weight, ride on a short road or parking lot so the pillion gets a sense of how it feels to ride and so you get a sense of the added weight. Practice slow turns and emergency braking if you need more time to feel comfortable.
Tips for the Pillion
Lean with the bike, but don’t force it! What I liked to do was hug the rider so our bodies moved/leaned as one. Don’t squish the rider though! You should position yourself so that you are straddling the rider and there are a few inches from your crotch to their butt. I would focus more on keeping your legs tight to the sides.
You will feel a jerk backward when the rider accelerates, so brace yourself and prepare for take off. When the rider decelerates, you will also feel a jerk forward that will force you to push up on the rider, so try not to squish him too much.
Be cautious that if you move around, the rider can feel EVERYTHING. If you are going to adjust your sitting position (especially if your ass gets numb), do so at higher speeds. Don’t adjust yourself when the rider is going slow on the street, wait until he picks up speed to adjust. It’s less noticeable for the rider this way.
Remember to pay attention to the road as well, just don’t be a backseat driver. Let the rider focus on the road. He doesn’t need more distractions if you’re waving around trying to take a selfie (don’t do that either).
One thing that was stressed a lot as a new rider was not putting all my weight on the foot pegs (like if you are pushing down on them). Sit comfortably with your feet on the pegs but don’t push them down. Don’t ever touch the floor, ever!
Wear appropriate gear and carry a backpack with a change of clothes if the weather is going to change. Thick jeans, sturdy boots, a motorcycle jacket (or thick jacket if none is available), and of course a helmet. Dress for the slide, not the ride (pretty grim, I know, but always be prepared).
Got any other tips? Comment below!