I cut a bunch of slits into the knees of my first pair of ripped jeans, and then washed them to wear at the holes. The holes got worn in over time. On my more recent pair of ripped jeans, I rubbed sandpaper over and around the knees - it did a pretty good job, plus the wear around the holes made it look like more realistic wear. And on both pairs, the holes at the knees have expanded a little bit over time (the former pair more than the latter).
Meanwhile, my pair of jeans that I wear when I'm building stuff or working in the forge remain unripped. Stained, yes, but whole. Hmm.