I watched an old-timey movie awhile back that had shirt stays in it, and I remember thinking vaguely about them. I went to an ROTC banquet the other night and everyone was complaining about their shirt stays, so of course I googled them and I just think they're the best.
Shirt stays look like a suspender/garter type combo. Another word for them is shirt garters. They attach to the bottom of your shirt and the top of your socks and keep your shirt tucked in. They're primarily used in the military these days. And boy howdy do they keep the shirts tucked in. I've been told there is also a stirrup variety. Google image "shirt stays", some amazing illustrations are provided.