I can't do anything better than advice your reading the following:
- J.A. Gallian, Advice on giving a good powerpoint presentation, Math Horizons, April. 2006:25-27
(It is a list of dos and don'ts, short and sensible; of course the same holds for OpenOffice and LaTeX slides, and most advice - also for classical blackboard.)
Additionally, it helps to watch people who are good at giving lectures/talks. One great event is TED.com (surely worth watching). While there are no research mathematical lectures - art of performing a talk, conveying ideas, focusing people's attention and emphasizing the main points is generally crucial.