“Dude, why would you ever crouch-walk?”
This is ancient, ancient advice, but it's something that I still occasionally see players doing on casual servers. Crouch-walking around a corner will always grant advantage to someone who's watching that corner—they'll always be able to see your gun poke out, then your knees, and blast you before you see them. If you need to check around an object, shoulder peek: dart out of cover while revealing as little of yourself as possible, then back in as quickly as possible, purely to see where an enemy is.