- Upgrade to PowerPoint 2007, files are a lot smaller, but many of my (corporate) clients do not have this software yet, so you end up saving in the 2003 format anyway
- Compress pictures: select the picture, in the format menu pick "compress" and select the appropriate DPI rate. I personally don't use it a lot: it does not save that much space if you use larger images, and the quality of your source file deteriorates forever. A big issue, especially when putting your presentation on a big overhead screen.
- Zipping files does not have a big impact when using images
What I end up doing is