I'm a huge introvert, and find social situations hard work. But weirdly, I love presenting for work, and am apparently very good at it. It helps that I am fanatical about my subject.
I keep my powerpoints very simple - they are there to summarise and keep me on track, not as entertainment. Cos thats my job 
Def fake it till you make it - great big smile when you get on stage, thank the moderator enthusiastically, and smile at the audience.
Move around if you have a radio mike, and look at the audience - move from left to right, front and back.
You can def over rehearse imo - they can look very false.
And for short talks I hate the 'tell them what you are going to tell them, tell them it, then tell then what you told them' approach. Takes up valuable time, and is clunky. Don't say your title either - people can read that. If you were talking about images of motherhood on internet forums, you could take time saying that or get straight in with 'You might wonder why this matters, but in our virtual world, the portrayal of women is diversifying etc'. Much more engaging