The biggest thing I see, as a bartender, are bartenders who don't pay attention to their customers. As long as you are not super weeded you should know who is going to need: a drink, their bill, you to stop them from committing various criminal acts etc. Also if you love cocktails and making good drinks then prepare to hate life if you find a job at a place that is not cocktail centric. I also agree that you should be giving people 10 in change with a five and five ones. I had a few old school bar customers set me start on that many years ago.