a piece of sturdy fabric that is larger than the hole (I used scrap pieces of denim)
dark blue thread
a sewing machine (I'm sure you could do this by hand, but I use a sewing machine in this tutorial)
Turn your jeans inside out and cut your patching fabric to size. I made mine a good deal larger than the hole to ensure full coverage and to provide extra support. Pin into place, then turn your jeans right side out.
Sew over your hole and patch tons of times (be careful not to sew both sides of your jeans together!). I used a tight zig-zag stitch perpendicular to the direction of the hole. Make sure that you go far enough in both directions to get to sturdy fabric. Seriously, just go back and forth, and back and forth, and back and forth. When you're done, you'll have a sturdy section of stitches.
Turn your jeans inside out again and remove the pins. At this point, you can either trim down the excess fabric on the patch OR sew down the edges. I chose the latter, figuring that if one area wore through then it could probably use the extra supportive fabric. Then, you're done!
While it looks pretty noticeable in these pictures, when I'm wearing them its really hard to see, especially if you have closer matching thread than I do. I've used the same method to reinforce areas that are wearing thin (soon to be holes) and have found those to be much more subtle as well. I'm just so glad to get some more wear out of these jeans! How do you breathe new life into wearing-out clothing?