Rands has written some really great articles on how to give and prepare for seminar presentations (too lazy for links; use Google). Here’s another gem of his via Twitter:
If you can’t practice the hell out of your slides, you can, at least, care the hell out of your slides.
I believe the point here is that with carefully-done slides, half of your problem is solved already. When your slides don’t help to pull you along in your train of thought, it’s easy to lose momentum and lose the coherency of your overall “message”.