If your PowerPoint files seem to grow, try cutting them down to size. One way to trim the kilobytes is by adding a blank slide (with the default design template) before the first presentation slide. Contrary to logic, this actually reduces file size. Why? Because the blank slide is used as the preview image that PowerPoint uses when you select a file in the Open dialog box. Because the preview image for a blank slide is well, blank, the file size is actually smaller.
When inserting graphics into a slide, consider optimizing the image. You can reduce the number of colors used with an image editor such as Adobe Photoshop, Macromedia Fireworks, or Microsoft Photo Editor. For example, if you have an image that only has red, white, and blue, you don't need 32-bit CMYK palettes.