Build a seminar around the main points you want to convey. Make sure that each one is clearly, slowly and explicitly stated when it first arises during the talk. At the end of the talk, restate all of the points in a summary.

Here are some additional principles, practices, and tips for preparing and delivering seminars. Some of these ideas apply to writing papers, too.

1. Assume your audience is infinitely ignorant but infinitely intelligent. The audience will understand a clear, complete explanation. (From Barry Honig.)
2. Give a good introduction.  A research report will be boring to everyone except an expert in your area unless you provide the context. Explain why what you are doing is interesting and important.
3. Never underestimate a person's pleasure in hearing a good presentation of what he or she already knows. Don't feel foolish providing background that your audience might be familiar with. And don't assume your audience knows a great deal more than you do, even if you are a graduate student and they are professors. They still need you to orient them to what you are doing. {From Andy McCammon.) 
4. Keep your audience oriented. Make sure that they understand, at each step, why you are presenting each topic. If the audience becomes disoriented, you will lose their attention. Presenting an outline of the talk near the beginning can help. (From Marti Head.)
5. A talk should entertain. The best scientific talks entertain by giving people new ideas and perspectives. High-quality graphics can also help.
6. Present only one big idea per slide. This helps convey your ideas forcefully, and prevents the audience from reading ahead instead of listening to you. (The exceptions are the outline and summary slides.)
7. Never read aloud from your slides! A slide should support your talk, never substitute for it.
8. Avoid spending time on highly technical points. Unless it is a critical detail, it is acceptable to briefly state what was done and that it was reasonable, and add that you'd be happy to explain it in detail if anyone is interested. Then continue with the seminar.
9. Practice. Stand in front of a seminar room and present to an imagined audience or to a friend. Identify parts where you have trouble finding the right words and work on them.  Make sure the talk is about the right length.
10. Benefit from other people's opinions of your talk. Parts of your talk may be less clear than you imagined. Even invalid criticism can be helpful: if one person thought your talk had an error, someone else probably will also, so head off future concerns by add a sentence or two to make your point clearer.
11. Pay attention to your audience when you're speaking. If everyone is watching you and listening, you are succeeding! If people look lost or somnolent, you may need to slow down, speak more loudly, and/or explain better. 
12. When you are speaking, you are in charge. A talk can be badly derailed if you get into a discussion or debate with someone in the audience. If the discussion seems to have no end, say that you'll be pleased to continue the discussion after the talk and then resume.
13. Check the time occasionally during your talk. Adjust the level of detail and the rate of presentation so that the talk fits the allotted time.