One of the most common clothing repairs I am called on to do is patching up the seat in jeans. If you cycle at all or, like me, have ~thunder thighs~ the fabric right in the crotch of your jeans have likely worn out before. It's a high stress area for pants in general. Combine that with how tight today's jean fashions are and the devastatingly low quality of today's denim and you're sure to wear your jeans out in a few months at the most.
(A Big Pet Peeve of mine with regards to the garment industry is around denim. We've managed to take one of our best fabrics, created to clothe hard-working labourers in comfort and quality, into something so flimsy and adulterated with synthetic fibres that it is nearly useless. Yes, stretch jeans are nice, but at what cost?)
Luckily, this is a fairly simple repair.
Be sure start with freshly washed jeans. This is important because even a few hours of wear can stretch the fabric out, and since we're working with pre-shrunk patches, we want to make sure the jeans are also pre-shrunk. Otherwise, your jeans will shrink a little in the wash and make the patch bubble uncomfortably.
Also, it's just rude to hand someone your dirty jeans to be repaired.
First, I find some scrap denim that matches (or sort of matches), and cut out some patches to appropriate size. It is important to make sure the patches are large enough to cover the holes or worn areas all the way out so you can stitch on solid, good fabric. Otherwise, your patch will rip off with only a few wears. I then finish the edges of the patches so that they don't fray with wear and wash. I use my serger, but you can always use a wide zig-zag stitch.
Then I pin and sew the patches onto the jeans, inside out. Again, make sure to sew onto good fabric. I often have to sew over the inner leg seams, as they too get very worn.
Next, I turn the jeans back to the right side and cut out the worn fabric, a few centimetres from the new stitching.
Finally, I zig-zag stitch all around the patch. I make sure to use the widest zig-zag with a very short stitch. This way the stitching is stronger and will last longer. If the jeans are thinning but not quite forming a hole, you can make the decision to keep the original fabric and have the two layers to guard against holes. I usually then do a little bit of straight stitching over the layers to hold them together.
And there you have it! (Nearly) good as new jeans! Sometimes the patched areas will feel a bit "diaperish" at first but that will quickly go away as you wear them.
I hope this helps! Let me know if you give this a try and if you find other tips or tricks you like to employ.