Party Hopping
Depending on your location or schedule, you may be inclined to party hop, especially to different house parties. Unfortunately, save for the champagne that is served at midnight, drinks might be scarce. I have been to many parties plenty of people show up without alcohol, and a party can certainly suffer without enough drinks. The beauty of a flask, however, is that you can waltz into any party, sit on a couch, chill with people, and take occasional swigs from your flask. And if there is plentiful alcohol at a party, you can even fill up your flask! Although I would recommend to do that discreetly.
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Another good aspect to party hopping with a flask is drinking along the way. If you're in a cab, on the subway, or walking, you have your portable alcohol whenever you feel like drinking. It doesn't get better than that.
Drink Specials
Many venues have drink specials for New Year's Eve. Obviously, they know people want to party hard, and they make sure to get the most out of that. Many places will offer an all you can drink deal, which is great- but you absolutely pay for everything you drink. I've found it better to avoid the all you can drink deals for this reason, as most venues charge extra on New Year's (in excess of $50). Also, most of the all you can drink deals involve one type of drink, and you can be sure the drinks will be watered down. If you have your flask, however, you can walk in without paying a dime for any drink special. You can order a few drinks, drink from you flask, save a TON of money, and get just as drunk, or even more so than most people. Also, because everyone else will have unlimited drinks, you can be sure you'll have plenty of swigs of free alcohol.
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In The Street
Many New Year's parties end up spilling out into the street. And in big cities like LA or Manhattan, the streets will be packed with people the whole night. Of course, it is against any bar rule to take a cup of alcohol into out into the street, even if you paid for it. And, drinking from a bottle in the street is illegal. If you have a flask, however, you can certainty skirt both of these problems. Granted, drinking from a flask is technically illegal, but it is much less obvious, and rather easy to hide, especially if you are among throngs of people. What's more is that the people with you and around you will be dying for a drink- your flask will make you very popular, hopefully around midnight!
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So, if you plan to head out for New Year's Eve, make sure you plan well. Dust off your flask, fill it up with something hard, and go out and have the time of your life. Happy New Year's!