Yay, something I can help with! What I do is sacrifice my oldest, rattiest pair of pants to be the patches for the others. I have a pair now that I've been slowly cutting parts off of for years now. When I notice that an area is getting thin on a good pair, I cut a piece of fabric that is larger than the thin area, so you're anchoring it to sound fabric, not part that's getting worn. If you're hand sewing, you're going to want to put the patch on the outside and sew around the edges. If you have access to a sewing machine, you can make your fix blend in a bit better. Put the patch on the inside, then sew over the thin area and into the patch over and over, going horizontally and vertically. The patch will give stability, and if you've matched your thread color well enough, the threads going up and down and side to side will be less noticeable than a patch that's just been sewn around the edges. The main thing is to make sure your patch is plenty big, so you don't end up with a new hole growing at the edge of the patch. Also, as the old saying goes, "A stitch in time saves nine." So if you do your mending while things are getting thin, but before holes appear, the whole thing will look better.