I've said my talk once in my head. Is that enough?
Rehearse, rehearse, rehearse! We can't stress this enough. Rehearse until you're completely comfortable in front of other people: different groups of people, people you love, people you fear, small groups, large groups, peers, people who aren't experts in your field. Listen to the criticisms and rehearse, rehearse, rehearse. If someone says you sound "over-rehearsed," this actually means you sound stilted and unnatural. Keep rehearsing, and focus on talking like you're speaking to just one person in a spontaneous one-way conversation.