Personally I got a pair of Skydex knee pad inserts from SKD Tactical, and put them into the knee pad pockets of Propper pants (because those are the pants I have.) They were like $20, and so far they've been working great. I really hate wearing knee pads, straps digging into my legs, refusing to stay anywhere near my knee, pants riding up as you run...
So this way I can have the (necessary) protection of knee pads, without all the things that make me hate knee pads.
These:
http://www.skdtac.com/SKYDEX-Knee-Pa...-p/sky.104.htm