See some have straps and some you just slip over your arm that are elastic (like a sock). I currently use one that is like a piece of cardboard with three velcro straps. I had tried a style that you slip over your arm but I wasn't able to get it over my arm. They are supposed to fit tight but this was too tight.
Interested in hearing what others use. I know some people don't have the problem with hitting there arm and I don't shooting with a t-shirt or sweatshirt but when I start wearing heavier clothing I worry so I use an arm guard.
I actually use military boot socks to hold my jacket tight to my arm and if i harvest a deer i can take it off to clean my hands and knife. Im serious.
if you are using one because you hit your arm all the time, then you are holding your bow wrong.