The Party: Some girl you don't really know is having a birthday pre-game
What to Bring: Split a handle of liquor with a friend or two
Why: There are a lot of factors to consider here, but if this is potentially a new group of girls that you and your friends could infiltrate, you've gotta make a good impression. Taking slugs of Rumple Minze from your flask in the corner is likely not going to make that impression. Make a real show of gifting the handle to the birthday girl as a "present", before immediately taking it back once she's distracted by all her birthday girl-ing, and drinking it yourself.
The Party: You're visiting your younger brother at his college and attending a house party
What to Bring: Liter of top-shelf booze
Why: No, you cannot get away with mooching off 21yr-olds. This is a great opportunity to show off in front of children who still suckle from the teat of plastic liquor bottles. Once the college kids see you strutting around, good stuff in hand, the whispers of "Whoa, Greg's brother must be doing really well for himself since he graduated" will commence. Even if your brother isn't named Greg, they'll say that. College kids call everyone Greg these days.