Plan well in advance. For an important presentation start preparing at least a week in advance. Write a first draft then leave it overnight before working on later drafts. If possible try to have everything ready a few days before you give the presentation (except perhaps for fine tuning).
Know your audience. To whom will you be speaking and at what level?
Decide what you would like the audience to learn from your presentation. Choose a small number of key points and structure your talk around them.
Keep the presentation as simple as possible. It is a common mistake, especially with scientific talks, to overestimate the audience and make the talk too difficult or too technical and this can have the unwanted effect of boring or frustrating most of the audience. On the other hand, it is often a good idea to address the experts in the audience for part of your talk, just don't lose the non-experts for too long.
Organize your material carefully. Usually it is essential to begin with some introductory or background material to prepare the audience and to explain the motivation for the work. The entire talk should be clearly and logically organized. Differentiate between main points and secondary issues. For longer talks a summary or a review of the key points at the end of the talk can be very effective.
Design your visuals carefully to effectively convey your message. There is a wide range of "effective" visuals, depending on the style of the speaker, the content of the talk, and the audience. Some speakers use very sparse visuals effectively, filling in gaps verbally, while others use visuals which are essentially complete and can be understood independently of the speaker! For most speakers something in between these two extremes is appropriate; use whatever best suits your style of presentation. However, as a general rule, complicated visuals with a large amount of information and/or a high density of information (such as a large page of text in a small font or a very dense plot or figure) should be avoided. On visuals "less is better" - presenting a small number of key points (say, one to three) per page works well. If you must show a complicated visual, make sure you tell the audience what they are supposed to learn from it.
Practice your presentation, preferably in front of friends or family, and ask for feedback. Check the timing of your talk! Does it fit comfortably within the time allotted? Running out of time is a common problem, and can ruin an otherwise good talk.
Think about the things you find especially interesting or exciting in your material and find ways to convey your enthusiasm to the audience. If you seem interested in your material your audience is more likely to be.
Try not to be too anxious about giving the presentation. Remember that most people feel nervous about giving presentations, but most audiences are well aware that giving a presentation can be stressful and they are forgiving of errors. Often only the speaker knows when there has been a slight omission or a minor change during the talk. Lots of rehearsal before the presentation helps to reduce anxiety.
Speak clearly and with sufficient volume to be heard throughout the room (this applies even if you are using a microphone).
Face the audience and establish eye contact, especially during the most important parts of the presentation. Some people find it helpful to identify a few friendly faces in the audience. Often a person in the audience will nod, smile, or give some form of helpful feedback.
Try to avoid mumbling, seeming nervous or confused, looking away from the audience for long periods, or generally giving the impression that you would rather be anywhere but here giving the presentation (even it it is true!). It is also advisable to avoid too many colloquialisms or an overly casual conversational style. For technical talks a slightly more formal style is common and (partly because audiences have come to expect this style of presentation) is more likely to convince the audience that the speaker knows the material.
Remember: whenever you make a presentation you are also presenting yourself. If you present your ideas clearly and persuasively, with self-assurance, skill, and professionalism, you and your presentations are likely to be much more effective.
Think through the presentation: which parts do you think went well and which parts would you improve if you gave the presentation again?
Ask a member of the audience (preferably someone you know well) for some feedback on your presentation. Viewing a video of your presentation, if one is available, can be another way of obtaining useful insights.