Do all the required reading for the seminar, and see if you can find the recommended secondary reading to go through that as well.
Write a list about what elements of the reading strike you as most interesting and worth explaining to the rest of the seminar group. The subject may require the coverage of certain material in your presentation, but it's always advantageous to expand on a topic which personally intrigues and stimulates you. This helps to make the presentation more dynamic. People can sense when you legitimately find pleasure in your topic and it can make them more attentive, improving the reception of your presentation.
Do additional reading and research on your particular interest area. If possible, use information sources which the other members of the group might not have seen. This can demonstrate that you're particularly enthusiastic about the material, and can also inspire other members of the group to look beyond their regular resources for additional information.
Sort your presentation into logical steps. Find the "narrative" of your presentation, and think about what sort of story you're trying to tell. This gives the presentation a sense of energy and life, and can make it much more interesting than just a collection of dry facts.
Implement tools to assist your presentation. Students will often use PowerPoint to illustrate their presentation. Choose the most appropriate presenting styles for your work.