Simply identify the main points of the speech. Cover them in the same order that they will appear in the body of the presentation.
For example, the preview for a speech about kites organized topically might take this form: "First, I will inform you about the invention of the kite. Then, I will explain the evolution of the kite. Third, I will introduce you to the different types of kites. Finally, I will inform you about various uses for kites." Notice that this preview avoids digressions (e.g., listing the various uses for kites); you will take care of the deeper information within the body of the speech.