This quote applies to telephone sales people, but it is relevant to presentations as well.

When a presentation is still fresh, the presenter clings on to her slides which contains the flow of her presentation. Good presenters get away from stage 0 (reading the slides aloud) and can fake spontaneity by really knowing them by heart. The next level up though is feeling so free and immersed in them that you can almost leave them out al together.

A bit like a guitarist, play the song, memorize the scale and improvise, and in the end, just flow with the music.