Groups are Good
A group can take on a bigger topic than an individual. Most students enjoy working in groups. In addition, standing in front of the class with your group is less scary for most kids than standing up there all by yourself. If you'd like more tips on groups, here is a post on making group work, work.
Be Clear about Expectations
Students should know what they need to cover, how long their presentation should be and what other elements they need to include such as visual aids, discussions questions, charts or stats, etc. You may even want to give your students a presentation structure for some subjects.
Teach Presentation Skills
Teach and practice presentation skills such as making eye contact, speaking at the appropriate volume and speed, and using visual aids or technology within the presentation.
Make it Interesting
Require some kind of attention-getting opening. Encourage creative approaches such as acting things out, presenting in song, or artwork. Students can bring in real-life examples, and visual aids. If you have technology - PowerPoint, for example, encourage students to use it.
No Passive Listening
The rest of the class should be learning from the presentation. Require the presenter(s) to come up with a short quiz about the presentation that the entire class will take. A Q&A session at the end of the presentation is also not a bad idea...after all, the presenters should know their subject well!
Skip the Peer Grading
Some teachers have students fill out grading rubrics for their classmates' presentations. Social hierarchy in school is already complicated enough without having students grade each other. As a parent, I would rather have my child take a test on the content of the presentation than grade the presenter. I am, however, in favor of students evaluating themselves. A self-evaluation and a teacher evaluation is enough.
Practice Makes Perfect
Do presentations often so that students have an opportunity to improve. Consider video taping presentations for students to watch on their own.
Presentations skills are a valuable commodity in today's world. The skills you teach today could be utilized in a riveting board meeting, a heartfelt acceptance speech, or a touching wedding toast tomorrow. Who knows, perhaps someday those skills will be put to use in a brilliant State of the Union Address.