"Don´t be seen" is an order, an imperative sentence. "Don´t get yourself seen" is more polite, the "get" gives you some leeway, almost as if it was some suggestion.
Other polite way would be "please, don´t let yourself be seen", anyway the "please" don´t mean that you don´t have to do what you were told (depending on whom is "asking" you to do).
You should get yourself a drink suggests, to my thinking, that the person is being somehow friendly to you. Whereas "you should get a drink" would sound like an imperative sentence, said by someone that is not a peer and want to keep it formal.
But then, it´s me here. My impression. Let´s wait for further clarifying and comments.