Rafting a waterfall is the whitewater equivalent to running full speed into a concrete wall. No matter what happens at the very least you know you're going to take a hit. It's not soft, there's no real graceful way of doing it, and worse yet you have to trust the equally intelligent person next to you that they aren't going to freak out or freeze up and put you both in even greater risk.
Here are some marginally helpful tips on things to do while rafting a waterfall.
Watch for flying objects.
If you and your buddy botch the entrance of the falls and you go off sideways, at least one of you is going to be launched. Falling 30' and landing in the raft is going to be jarring. Now imagine falling 30', landing in the raft, and having your 200 pound paddle partner land on top of you and hockey check you out of the boat. That's a lot of force; you might as well do your best to dodge it. If you're going to be on the receiving end of gravity try and get down as low as you can in the boat as it makes contact with the base of the falls. This way your friend will sail over you rather than into you. It may be a jerk thing to do but it's not your fault that they can't stay on their own side of the boat.
If roles are reversed and you're the one doing the aerials, try and stretch out as large as possible. You only have one shot at not being the only one to swim so make sure it counts.
See a video of rafters on the White Salmon River in WA: Green Truss Section.
While paddles are useful for going off the falls straight, at some point during the freefall they transform into dangerous pain inflicting instruments seeking out teeth and other precious body parts. Ideally you want to keep the paddles as far away from one another as possible. One technique you may want to try is right as you go over the top of the falls reach over and swoop your friend's paddle from their hands. They'll be so focused on the ground coming up at them that they won't even see it coming. You'll have both the paddles and plenty of reason to smile.
Take advantage of a bonding moment.
There's no better time than while in freefall to have a heart to heart with your paddling partner. Feel bad about spilling beer on their computer and that's the real reason it won't turn on? Were you really the one who let their dog out just prior to when it got hit by a car? Well now is the best time to get it off your chest. It's probably the one opportunity when they'll be so preoccupied with the events at hand to really even get that upset with you. Chances are they won't even be listening so tell them to their face and get off scot-free, trust me you'll feel a lot better afterwards.
It's never too early to celebrate.
Once you pass that last eddy you've committed to going over the falls, why wait till after the fact to start cheering about rafting a waterfall? Since your friend won't have a paddle in their hands anymore, reach over and slap them a high-five or pass them a cold one. Regardless of what happens when the raft crashes into the water at the bottom at least you'll look like a happy-go-lucky person seconds before.
Best of luck and remember no matter how much of an expert someone says they are on "Hucking Rubber", truth is all they've really done is mastered the ability to "Huck and Hope".