If you are falling off of the slider on one side, you can do one of two things. You can change the angle that you take to get on to the slider. You may be cutting with too much or not enough angle. The other thing you can do is shift your weight to the side opposite of the way you are falling. If you start to fall off of the right side of the slider, shift your weight to your left foot and vice versa.
Slower speeds make rainbows MUCH easier.
Pull first timers at about 15 to 17 mph. Use your brain and common sense when building sliders and always wear a helmet when hitting any kind of obstacles. Safety should always be your number one priority. If your wakeboard is your most prized possession, don't slide it. Honeycomb boards are even more prone than foam boards to denting on sliders.