The advice of knowing your material really is extremely important. Memorizing what you have to say generally comes across as forced and unnatural. I generally like to have an outline of what I want to say, perhaps with some notes to make sure I don't skip over a the major point, but other than that I make sure I know the material well enough that I can talk about it casually. This is especially important if people may ask questions in the middle of your presentation rather than just at the end (if at all).

There is the extreme where you memorize and script it out and practice it so well that it comes off as totally natural, but I have yet to take it quite to that extreme.

Practice your presentation alot, in front of at least one other person. It will make the specific presentation better and your general presentation skills will increase as well.