I often find holes wear in my jeans between the legs, so I patch them with fabric on the inside and lots of rows of zig-zag sewing.
It's not a very visible spot so it does not have to be perfect, but it definitely increases the useable life of the jeans considerably.
You can also do the patch before the hole actually wears through - even just stitching over the top of the thinning area is probably enough to provide extra reinforcement to stop the hole forming, although the extra fabric provides a more stable base for the new stitching.