This might sound really stingy but alcohol is good to share in the first few days. After that they can buy their own, I don't fund anyone's drinking habits.
My current flat mate is into buying rounds, which I don't mind if there's just a couple of people but when there's five or six it gets expensive. I usually drink a fair bit before I go out so only buy one drink.
EDIT: I only buy drinks for one person if we go out because in the grand scheme of our relationship, I probably owe him anyway.