A Concise Guide to Tray Surfing

Required equipment:
-One car. An oh-shit handle above the driver-side door is preferable but not necessary.
-Two plastic meal trays, the kind you put your dishes on in a cafeteria. Use your own if you (for some reason) have any, or beg, borrow or steal them from your local fast foot joint, cafeteria, or dining hall. Make sure they're the plastic kind; the fiberglass variety offers less traction, makes an extremely foul odor under pressure, and probably gives you cancer to boot.
-At least one cohort; someone will need to drive the car.

Find an isolated, fairly flat stretch of road. You want a minimum of sharp curves or changes in slope, and the pavement should be in reasonably good condition. Parking lots work well, but watch out for speed bumps.
Have your friend start the car and roll down the driver-side window as far as it will go.
Stand just outside the driver-side door, with the trays nearby. Grab onto the car's inside door handle; grab the oh-shit handle as well if the car has one and you don't think it'll fall off under stress, otherwise use both hands to grip the inside door handle.
Get in a low stance, with one foot on each tray. Your feet should be flat on the trays and slightly ahead of your body; your knees should be bent, and one leg should lead the other slightly. Your stance should be fairly secure; if you feel about to fall, adjust yourself until you don't.
Now the fun begins. Have your friend get the car into low gear and start driving, with you hanging off the driver-side door. It's not a good idea to go faster than fifteen or twenty miles per hour (~35-45 kph), and about half that is a good speed on rough pavement or over speed bumps. You can stay secure at up to about thirty-five miles (~70 kph) per hour, but I don't reccomend it; losing your grip or your trays at that speed would not be anywhere near pleasant.
Night is the best time for this; the pavement is cooler, meaning that your trays won't heat up as fast, and it's harder for people to see you. Avoid crowds and cops; I don't know of any specific laws prohibiting this activity, but I am sure that They could find something to bust you with. Be aware that it makes plenty of noise.

If you are surfing:
-Put on some old clothes that can stand a bit of road rash. This isn't strictly necessary, but you'll thank yourself for it if/when you wipe out.
-Don't go more than about half a mile at a time. The trays heat up very quickly; if you put too much stress on them, you could melt through them. This is a Bad Thing.
-If your driver is going too fast, you can tell him to slow down or just lean over and smack him in the face, whichever you feel more comfortable with.
-The trays will eventually start to slip on your feet. This is normal. If your feet migrate to the edge of the trays or near the edge of the trays, however, have your driver stop and reposition them; for obvious reasons, you don't want to lose a tray.
-If you have a morbid aversion to pain, find something else to do with your night. You will probably wipe out eventually, though maybe not in your first night of tray surfing or your second.

If you are driving:
-Do what the surfer tells you to, unless it's patently stupid. This goes doubly for instructions like "stop" or "slow down".
-Do not go faster than the speed limit or about 30 MPH (65 KPH), whichever is less. Half that, around corners or uneven pavement.