First of all, if you can move your feet and get under the ball, you should never dive.
When it is hard to get to the ball, for boys, they dive with arm/palm support, and land on their chests if ball is "in front of them" or land on their knees and hips when side-diving.
For girls (and older boys including me :), they should land on knee and hip, and quickly roll to their back, no matter which direction they are going. See the following link for more details regarding "Dive and Roll Over to a Standing Position":
http://volleyball.about.com/cs/advancedinfo/ht/Dive_Roll_Stand.htm
I believe it was some talented Japanese coaches (and their female players) who invented the technique, and got their three-peat with their outstanding defense, then Chinese women's team learned it and had their five-peat...
It is a good idea to have special shorts, or a pair of thinnest baseball sliding shorts inside of your volleyball shorts (if possible). I personally use sliding shorts (inside my basketball shorts so that nobody could tell) these days and they helped a lot.
By the way, I have played 20+ years against players at different levels, and never had to repair my knees or hips... so don't worry that much.