To be concise, talks must be carefully planned and rehearsed. It is impossible to overemphasize the importance of rehearsing. The more times you go over your talk, the more confident you will feel when you stand up in front of an audience, and the more convincing and informative you will sound. Rehearse for your colleagues, your room-mates or friends, your cat or dog, yourself. The most effective talks are conversational, but not colloquial.
Talks delivered in very formal language are hard to listen to.
Talks that are filled with slang are not taken seriously. And if you sound interested in your work, your audience will be interested, too. Don't try to say too much. Think about two or three things that you would like your audience to remember a week from now, and structure your talk around those points. Usually, they are conclusions, but one of them maybe a new technique, or a really exciting result.