Always take your empty glasses up to the bar when you're leaving the boozer. Don't leave your mates nursing glasses of fresh air, get your round in before they finish their pints, when they're down to the last few gulps. If someone has been waiting to get served for ages and you get the nod from the bar staff when you've just arrived at the bar, let the person that's been waiting ages get served first. Don't let your mates buy you drink all day and then make some pishy excuse not to get your round in.
Any others that you think must be adhered to?