How should I decorate for my husbands going away party?

Now that i have nailed down where I am having it, which is at the American Legion, a veterans orginaziot, banquet hall, I need to know how i should decorate. My husband is in the Marines and leaving for Afghanistan early October I am throwing him a going away/24th birthday party mid September and I have never ventured into decorating for a nice party. I want this to be a dressy party, nothing too formal just dressy, but casual so dresses for girls collared shirts for guys, stuff like that. Also, I am having the party catered pretty nice with Hor'd orves and there is an open bar and DJ. SO for decorating this empty room....what to do?!?!?