Do not be that guy who staples a piece of cardboard to his pocket square, in order to have it stay in place better in his pocket. (I've actually seen pocket squares sold this way - stapled to pocket-shaped pieces of cardboard, and meant to be worn like that.) A stapled pocket square is... is... well, I don't know how to accurately describe just how wrong it is. Maybe you could think of it as being pre-tied necktie wrong.
That's a brilliant idea!!!
But seriously, I understand it won't stay still because it is silk but thought I just ask in case someone knew of a good way to keep it still. I am a 38R and I use the same pocket square on all my jackets so it aint the jacket, it aint the fold, it's just that it is silk. At least it doesn't fall in when I am wearing it.