There are many different pieces of clothing and items you can decorate with patches. While most bikers add patches to their jackets and vests, you can also add them to your pants, canvas belts, cloth bags, saddlebags, and even baseball caps. While you can't add every cap to your hat, there are a number of specially designed patches that are perfect for baseball caps.
These special patches are almost like a half circle. The bottom, however, is not usually flat but is angled on either end. This patch is designed to sit in the center of your cap in the front, right over the bill. Some other patches can be put on the front of your cap, too, of course. Since most hats are made out of canvas, not leather, you can iron the patches on using the heat-activated plastic adhesive on the back of the patch. You could, of course, sew the patch on if you wanted, but it's not necessary. While there isn't a huge selection of patches designed specifically for ball caps, you'll find that most small patches will fit nicely.
If you wear a lot of beanie caps, you can also add patches to them. Since they don't have a set front or back, you can actually put patches all the way around the cap if you'd like. You can also put fairly large patches on beanie hats.
Adding patches to your hat might seem like a small thing, but it can be yet another addition to your overall look.