My methods, depending on the group--some can be combined:
Get a pitcher for the table, first thing. Usually $7 to $20, and then somebody will usually buy the next one, someone the next one, etc. Or if it is a small group, the one pitcher, or two, is enough for the night.
Go to a brewery or bar that is BYOF--bring your own food. I'll bring in carryout from someplace cheap, or else food I made at home in a carryout container. More expensive, but better beer.
Order a water with every drink order, then order a water when you're done with both before you get another drink. Aside from being cheaper, it also gets you less drunk but still buzzed, and MUCH less of a hangover even if you do drink all night. Hydration, yo. (I do not do this in places with disgusting bathrooms)
Find out what the cheapest beer is, and just get that. PBR, Hamms, whatever, as long as it is cold.
I don't like the flask method. Frankly, that is slimy. I don't bring in a PB&J to a restaurant because their food is too expensive; I won't do that with drinks either.