In any election, there are going to be a particular set of hot button issues that are most on the minds of the voters. It is vital that you familiarize yourself with what these issues are. If you start droning on about things that only you care about, while ignoring the things the general student body is concerned with, you have almost guaranteed your own defeat. In addition, don't just talk about these concerns -- have actual suggestions for things that might be done to address student concerns.
There is nothing more likely to turn the electorate against you than to give a rambling, poorly supported speech that leaves half the audience not knowing what you meant. For this reason, you should carefully research the issues you are going to address to make sure you fully understand them. Once you have your research, organize it in the same way you would a paper. Supply facts and rational arguments to support your views.
It can't hurt to add a bit of humor to your speech. This can be a simple joke about the weather or the crowded parking outside. However, stay away from humor that might antagonize anyone. In addition, keep in mind you are running for a position on the student government and that you should treat the occasion with a measure of respect.
One of the worst and best things you can do when making your speech is to use props. On the one hand, if you bring charts and graphs that no one but Einstein could understand, or you have so many images and slides that you get yourself confused during your speech, you will quickly turn off your audience. On the other hand, a well planned set of props that you have carefully practiced using until you could handle it in your sleep can add interest and even support for your views.