This is an excerpt from Mastering Skateboarding by Per Welinder & Pete Whitley.
Wearing a helmet is the most important thing. The second most important thing is learning how to knee-slide out of a bail. Put on your knee pads and make sure they're comfortable but tight. You should be able to knock them around with your hands and jump hard up and down without them jiggling around.
This is the basic technique for knee slides:
1. Find a smooth chunk of concrete at the skatepark (or even your garage).
2. Drop heavily to your knees. The impact should be comfortable.
3. Stand up, run a few steps, and drop to your knees again. You should slide forward a foot (30 cm) or so. Again, the impact should be comfortable. You'll notice that you also slid on the top of your feet. Unless you're wearing thin slip-ons, you shouldn't feel the slide in your feet.
4. Repeat the slide a few times. You'll feel the pads tug at the ground over imperfections and rough patches in the concrete. Lean back and distribute your weight between your knees and your shoes.
5. Now take it to the bowl. Pump back and forth until you're about halfway up the transition. When you roll up fakie and are looking down the bowl, drop off the board and land on your knees, with your feet tucked under you.
6. Lean back slightly and slide down the transition.
You should also try a few practice bails while facing the wall and sliding backward. It's more challenging but is a great thing to know.
Some people complain that after skating in knee pads for a while, they forget how to run out of unsuccessful tricks. They instinctively want to fall to their protected knees any time they are bailing. When they take off their pads, they find themselves wanting to land hard on their unprotected knees. This is a better conversation piece than an actual risk. Any skaters who have gotten hurt by thinking they were wearing pads when they weren't may want to reevaluate their skateboarding hobby.
Read more from Mastering Skateboarding by Per Welinder, Pete Whitley.