The Bottle Trick
You have to have a drink, of course, or the bar staff will ask you to leave. The trick is to wrap a paper napkin round the bottle and to drink, or pretend to drink, through a straw. Waitresses can't see through the napkin, and the straw means you never tip the bottle. For all they know, it could be half full. One Heineken can last all night. With practice, your lip-work on the straw can draw more than beer. Showing the tip of the tongue helps.