I've given a few talks about my master's topic at a local, regional and national meetings (society for applied anthro, society for human ecology...that type of thing), and have given advice to other grad students who told me it worked great for them.
First step: like your topic. Sounds pretty basic, right? Even if you don't get to pick your topic, find something in it that you think is cool and interesting that you'd like to share with others.
Second step: practice just talking about your topic. Informally. Tell people about your topic every chance you get. Corner strangers on the street. Do this often enough, and you learn 2 essential things:
1. what people are interested in about your topic (use this to keep your talk interesting)
2. what you need to say to explain your topic
3. (very important) what you can leave out and they'll still understand your point. (don't overexplain once your audience 'gets it') - learn this by paying close attention to what questions people ask when you give very short 'sound bytes' about your topic.
This is a multi-stage process and works best for thse who work with the same topic over a long period, but you can corner friends and family members for 'short term' topics too.
relax and enjoy telling people 'cool stuff' about a topic you like.