Smile and the whole world smiles back. Never more so than when you are presenting. With the odd exception of having to deliver bad news, it is invariably better to smile. If you are nervous, look for friendly smiling faces in the audience and give them lots of eye contact to begin with, it will help to calm your nerves. Do not forget to look at everyone else in the audience as your presentation progresses.
By smiling, you look like you are enjoying giving the presentation, which in turn encourages your audeince to enjoy it as well.