The way you prepare your slides informs the way you give do the oral presentation. So try to visualize what you will say about each slide while you are preparing it.
Choose a font, a font size and a color scheme that is easy to read (see the video on the left).
Use text sparingly, figures and graphs are a much better way of conveying information fast.
On the flip side, using too many figures will confuse the audience -- find a good balance.
Label your figures and axes clearly.
Pay attention to how you start the talk. "Good morning ladies and gentlemen" is not a good first sentence. :-) Introduce yourself and say briefly what you will talk about.
Assume that your audience knows very little about your work if at all. The level of assumed preparation will of course depend on the type of talk you are giving. If you are an astrophysicist talking to other astrophysicists, you don't have to explain every single detail. But if you are talking to a general audience, do not assume what is obvious to you is also obvious to them.
Avoid too many abbreviations unless the audience is familiar with them.
If it is difficult to avoid complicated slides, emphasize to the audience what is the most important thing to keep in mind from that particular slide.
Look at your audience. Keep eye contact but do not fixate on a single person; make sure your gaze scans the entire room.
Avoid reading your slides. An oral presentation implies you are narrating your work; the audience can read your work from your papers/reports themselves. They are there to actually hear you talk about it in your own words.
Everyone has a natural pace and volume of their voice. The best would be to talk at an understandable pace and at a high enough volume -- however, these habits are difficult to gain if you don't happen to already speak that way. Learn compensating skills; if you talk with a soft voice, get a microphone and make sure that you look up so that your voice projects better. If you have a very fast speaking pace, talk in shorter sentences, take breaks and frequently ask the audience if there are questions to make sure they are following you.
DO NOT go overtime. The speed with which one gives the talk fluctuates each time you give it. For those cases when you find yourself running out of time, have a backup plan for skipping a few slides -- this means that you should present your most important results before you get to the end of the talk.
Don't overdo the pointer, use it sparingly. If you move it around too much, the audience will not know what to look at.
Answer the questions from the audience concisely, addressing the point directly. If the audience member starts to argue with you or antagonize you, politely acknowledge that they are raising a valid point and that you will be happy to discuss it later (you may not have to do this yourself if there is a moderator or a session chair of some sort). This will give other participants a chance to ask their questions before time runs out.