I was pretty bad with dropping my hands when I started... Best way to get used to it is to hold them up there when praciticing anything. Like when you start learing footwork & moving around your opponent, make sure you keep your hands stuck up there where they are supposed to be, even if you aren't learning any punching of defence at the time.... Try to live in that stance when your practicing anything in the gym & eventually they'll stay up there..

Of course your brain learns very quickly after you eat a few punches, so don't stress about it to much.