"A stitch in time saves nine." Reinforce that worn spot before the hole starts. There are different ways to do this, the easiest is probably to run some zig zags over the spot with a sewing machine. The best color for denim is GRAY THREAD since is shows up the least. If you want it to be more invisible, I like to take and needle and thread and run stitches through the diagonal "tunnels" created by the warp (navy threads). More time consuming, but it does a great job. Finally, if you have an actual hole, you can darn it just like you would with a sock (look up demonstrations on youtube). You pretty much stitch a circle around the hole, then stitch a lot of vertical lines then weave them with a bunch of horizontal lines. This is my favorite method because the thread eventually blends right into the denim, plus you're creating new fabric over the hole so it doesn't pull the jeans into a weird shape. I never liked patches since they often make a weird shape under the hole.