Green Jeans

I use these every time i garden, wish i would have had em when i was in masonary

http://www.landscapeusa.com/greenjeans/

You can also get workmen's trousers made of very tough fabric which have pockets over the knees for sliding knee-protectors into. I make my own out of high-density foam - actually an old camping mat. My other half uses them a lot at planting time. Without them he wears the knees through within weeks.