Creating a presentation

Your slides are not your talk
While it is tempting to spell out everything in full detail on the slides, remember that the slides are not your presentation. The slides are merely there to support your presentation.
Use pictures
In addition to the previous point: use graphics where useful. This makes the presentation much more lively, and offers much less of an invitation to start "reading the slide".
Rule #1: What do you want to say?
Be very clear: what is the point you're trying to make with this bullet point (or even: with this slide or with this part of the presentation)?
If you can phrase that succinctly for yourself, then you can explain it well in the presentation.
Rule #2: Why do you want to say this?
Now that you know what you want to say, ask yourself: why should the audience hear this? How does this fit in with the rest of the presentation?
Asking this question can help you to not go too deeply off on a tangent that, while interesting, ultimately is a distraction from the main message of your presentation.
Timing: 1 min / slide
You will want to know how many slides you will need to fill the allotted time, without going too much over or under. A general rule of thumb is to count one minute per slide. If you have a few slides more than minutes, you'll probably have to speed up at some point.