I never can get the iron-on type of patches to work for mending clothes. They start to come off after the first washing. But while trying to iron one on, the idea occurred to me that there ought to be iron-on "bumper stickers" for clothes. You could use them to mend the seat of your work pants or put them on the back of a t-shirt or cap - at least if your cap has a back to it.
I want one for the back of my cap that says "How am I thinking? Call SANE-THOUGHTS".