If you are a tradesman that needs knee pads on the job, I came across a product that I really liked. I was at a flooring tradeshow yesterday and came across www.proknee.com
I tried a pair on and I immediately noticed that they cannot slip down and they do not have a strap that will cut into the back of your knee. I only kneeled for a little bit, but seemed very comfortable. While standing they would not seem to move out of position. Plastic shell rests on the top of your foot so that it won't move down, and the pad is cupped to fit the knee and not move side to side or up. They also have a thick leather outer covering for long wear. It looks like they cost $180-$210. A bit much for a DIY'er, but for a professional, would be worth every penny if it saved your knees and were comfortable to work in. I'm more of a serious DIY'er and I'm seriously considering a pair when I have some extra dough. If you spend a lot of time needing that type of pad, that cost really isn't much at all if it means saving your knees and pain.
I am not affiliated in any way with this product, I was just impressed by it. I love when people build a better mousetrap.