Preparation

Once you have a general idea of what you want to say, you'll have to decide how to say it. Unlike a conversation or a written document, a talk is a one-shot attempt to make a point. By contrast, a conversation consists of repetitions and clarification's based on questions and immediate feedback, while a written paper allows a reader to puzzle through its contents as often as necessary. It is essential that your talk be well-constructed and tidy, and that your points be presented to the audience both a logical sequence and unambiguously. This all takes a fair amount of preparation. Start early!

Here are a few pointers to get you started:

Start preparing far in advance by thinking through what needs to be said. Collect material which may relate to the topic from unusual sources, and sleep on these ideas. The final product will be more fully-developed and interesting.
Using big letters and a bold pen, write a clear statement of the problem and its importance, and then pin that statement on the wall above your desk.

Develop this theme into one jargon-free sentence that will catch the attention of the audience. Next, identify the issues you plan to address (brainstorm, then trim back; see the portion of this tutorial on outlining).

Arrange these issues in a logical sequence (which may change as you develop the talk). This process is easier if you use index cards to organize your talk, with one idea per card.

Computer-based presentation programs (PowerPoint, Persuasion, etc.) can be wonderful time-savers. The time invested in learning to use these programs is rewarded by the speed with which a presentation can be created, even by a moderately-skilled user. These programs are good tools for organizing your presentation (an electronic version of the index cards idea), they can be used to create visuals for the presentation (e.g., slides and transparencies), and even project those visuals during the presentation.

Avoid using lists (First ..., Second ...); you may confuse listing systems (First ..., Point B..., and another thing ...), or you may discover later in the talk that you've missed a point entirely, and then you'll be forced to backtrack. Both of these problems tend to distract your audience away from the points you are trying to make, and both give the appearance of poor organizational skills.

Retention of information by the audience is reduced as a talk proceeds, so if you do want to make a series of points, organize them from the most to the least important. That way, the audience is more likely to remember the important points later. You may even find that the less important points become irrelevant to the focus of the talk as you practice.

Determine transition elements which will help your audience to follow the link from one issue to the next. These should be logical, and may presented by posing a question, or explaining your own discovery of the link's existence.
Use short sentences with simple constructions. The concept will be made more clear, and the sentence structure is more similar to conversational styles.

Run through the talk once, early. Go back and re-think the sequencing. Discard non-essential elements.

Don't assume the audience will be familiar with basic concepts that form the foundation of your talk. Outline these concepts briefly but clearly early in the talk to avoid confusion.

Attempt to identify problems or questions the audience may have and address them in the talk, before the audience has a chance to think of these things themselves.

Determine which elements would benefit by being presented with visual aids. Spend time working out the best way to present the material. Head on over to the accompanying tutorials for information on presenting material in an effective way using visual aids.

Prepare thumbnails sketches of these visual aids, then run through the talk again. Re-work the most appropriate and essential visual aids and discard the rest. Don't forget to proof-read your visuals! Do so while there is plenty of time to re-print that critical slide with the glaring typo.

The earlier you start on the visuals, the better they will be. On the other hand, avoid fine tuning each visual endlessly; if you find yourself diddling the details, go on to do something more productive instead.

When in doubt about which presentation medium to use (transparencies, slides, videos, multimedia, etc.), choose the format which is the least complex which remains consistent with both clarity and content of the presentation. Keep in mind that the more technology you use, the more things there will be which can go wrong. These technological difficulties may develop into a gruesome presentation experience, particularly if you are giving the talk in an unfamiliar setting!

If you do need to use multimedia technology in your presentation, call ahead to make sure the technology you require is supported in the room where you'll be talking!

The most important preparation factor is to REHEARSE! Do so in private at first. Then for a real acid test, videotape yourself and watch the results with a critical eye. It's often a painful and humbling experience, but the results will be worth it.
You can then try the presentation out in front of a few colleagues. Ask for feedback, then act on that information. Select those who know a little about your topic, and not those who know a lot. This will focus your attention on attempting to explain why you did what you did in simple terms, rather than encouraging attention to details only specialists care about.

If you start preparing early, you'll have plenty of time to refine the presentation based on your colleagues' feedback. This is always a useful process.
Don't waste your colleagues' time; if you are sincere about wanting that feedback, don't wait until the night before the presentation to ask for other people's input.

Remember, the shorter the talk, the more difficult it will be to cover the material clearly and completely. Be strict about including only what is essential information for the presentation, and removing all the non-essential tidbits.