When told to come in, walk in, march at attention, face chairman of board, "Sir LT (or whatever your rank is) ### reports as ordered" while saluting, get your salute returned, drop and take your seat.
At the end, stand up at attention, ask "Will that be all sir", salute, "Good day sir", march out.
Some boards take this less seriously than others (I can remember hearing the story from an AMS classmate, who was going to fly pointy-nose fighters, that his board started openly snickering when he did it). But it's better to err on the caution, and hey, have some pride. You're in the military, none of the civilian applicants are going to know how to do it. Good way to set yourself apart.