I do a lot to protect the knees:
1. Slider shorts
2. Smart Parts knee pads (I know, I know...SP sucks. But their kneed pads are great)
3. Pants with thick kevlar knees (e.g. Empire LTD)
I can think of at least two occassion where even with all this, I nearly tore off my kneecap. Anything less, I would have been in the hospital. So I err on the side of precaution...