If you've yet to enter the world of flask drinking, here are a few things to remember that will help you get the most out of what could be a long, beautiful relationship with the world's coolest drinking vessel.
The flask is as close as you can come to a foolproof gift. There's one in nearly every price category, they're easy to engrave, and even tee-totalers can fill them with a non-alcoholic beverage. Anybody who has one can always use another, and anybody who doesn't want one is lying, or is someone who should come off of your gift list. Permanently.
2.) Do fill it with whiskey.
There's no way this can go wrong. It's great at room temperature, great if you want to splash it over ice, and great in whatever you come across that you might want to spike (up to and including ribs). Now, that said ...
3.) Don't underestimate the portability of cocktails.
The classic choice is to fill your flask with a single, swiggable spirit, or something you can use to subtly amp up your own coffee or punch. But don't overlook the joys of carrying a perfectly-mixed Sidecar, Manhattan, or Old-Fashioned in your pocket -- just make sure to avoid cocktails with cream, soda, or egg whites or anything else you don't want coddled or flattened by your body heat. If it's something that will work best cold (a gimlet, say), and you'll be drinking it rather quickly, mix it and stick it in the freezer before heading out the door.
4.) Do expect to share.
You're going to carry a flask and not share it with your friends / fellow parishoners / other people standing in line at 5AM for discounted TVs on Black Friday? Don't be that guy. The flask is not for germophobes.
5.) Do mind your liquor laws.
Yep, even when it's screwed shut, that flask in your hand or your jacket or glove compartment is an open container. Those are illegal most places. Know before you go. (Don't even think about getting a full one through airport security, unless you enjoy long days in small rooms with the TSA.)
6.) Don't get precious about it.
It's a flask, not a thermos, a bra or a tea set. Don't do anything that would make Humphrey Bogart roll his eyes at you: No leather paneling, no tiny matching cups in your bag, and for the love of all that's cool and holy? No Ed Hardy.