Don't try flair until you feel comfortable doing so without failing constantly.
I saw so many new bartenders think they could flip a bottle and look cool doing so only to get laughed at continuously.
Friends stop being your friends when you are working. They will try to take advantage of getting free drinks because they know you. Ignore them. They probably won't tip you anyway.
Free drinks are for people who have spent enough money and have tipped you or that you know are good tippers already.
Avoid giving free drinks to women who are flirting with you. Once you give them the first one, they'll expect it the whole night... and trust me, if she wants a free drink, you probably weren't going to get laid by her anyway.
Long pour and perfect pour are your friends. Not only will they impress your boss, but it will impress customers as well. Also, a perfect pour will avoid loading up drinks. You NEVER want to load up a person's drink. This is actually counter-productive. This will only mean that they will spend less and leave less tip, most probably.
Card anyone, specially women. Like Amazing said, I had so many hook-ups just because I asked for an ID.
Learn how to make original drinks and learn how to recommend SOMETHING.
You have no idea how many times I had women come up to me and ask for a Cosmopolitan and I'd say: No, you don't want that Sex in the City crap.
I'd give them something to try that was similar but so much better.
And lots of times, they'll walk up and say: I don't know what to drink.
A savvy bartender impresses more and makes more tips.
And avoid talking about politics/religion.
Make sure you know about sports.