How do I keep from wearing out the knees on my jeans?

I haven't always been hard on the knees of my jeans, this seems to have started to develop over the past couple of years. Normally, my jeans wear out right above the corners of the back pockets (explain that one, if you can) but, as of late, it's been the knees.

I know that the lifespan of jeans can be extended by using proper care (washing in cold water only, not allowing them to soak for a long time in the detergent, not using harsh detergents, allowing them to "air" dry, etc), but it seems that I am blowing through jeans faster these days.

I've always been fairly "active" in my jeans, using them for everything from cutting the grass to attending evening functions (I'm not very fancy), but I don't recall any changes in my lifestyle that would lead to more wear-and-tear on the knee area.  What gives?