Pick a topic that is researchable. The more obscure your choice, the harder it will be to adequately develop it. Choose something thought-provoking, no matter the format, be it a convincing argument speech or an informative one.
Narrow your focus. For example, if you are giving a speech about steroid use in Major League Baseball, focus on testing measures the league and commissioner have considered and why or why not they should work. Covering a lot of information is commendable, but you do not want to overwhelm your audience.
Explore and reveal different angles on topic. As mentioned, you don't want to attempt to cover too much, but you do want the speech to be balanced. For instance, if you are saying, in the baseball speech, that current testing measures are inadequate, then also include a mention of what those opposed to your point of view say.
Use illustrations. Not only are they good for engaging the audience and preventing boredom, they are also useful tools to further develop your topic. This can be simplified by using simple pictures that you will use to explain your point, or you could use more detailed charts and graphs.