I know it's hard to be nice to people you don't really care for, but it's not that complicated. It's easier to be nice,when you are happy. Try talking softer, and don't be sarcastic. More importantly, think about what you are going to say, before you say it.
You can be nice to others by first showing respect to yourself and show others that they matter by giving them your attention. If you are in a bad mood, don't let it affect the way you relate to others or even talk to them. Get more tips from http://www.ehow.com/how_2068356_be-nice-others.html.