Don't do this with PowerPoint. Seriously.
In Do this before you use PowerPoint for anything serious we show you a few things you should always do before you start using PowerPoint in earnest.
There are also a few things you shouldn't do in PowerPoint unless you really, really know what you're doing and why. And even then, think twice about it.
Don't bounce presentations between 2007/Win or 2008/Mac and earlier versions
When you save from PowerPoint 2007/2008 to the earlier 97-2003 or equivalent Mac format, PowerPoint tries to preserve the appearance of your presentation at the expense of editability. It also creates lots of extra slide masters. This is fine if all you need to do is view the presentation in an older version of PowerPoint. But if you edit it there then open it in PowerPoint 2007, and especially if you move the presentation through several back and forth cycles like this, you may find yourself deep in weirdness. Headers, footers, slide numbers and dates are liable to appear in unexpected ways and you'll find a whole lot of unwanted, unneeded masters and layouts.
If you're collaborating with others, move "preview" versions of the presentation back and forth as needed, but confine the "real" editing to one file in one or the other of the versions, not both.
Don't bounce presentations between Mac and PC
See above. While the problems are slightly different, repeatedly moving a presentation between Mac and PC versions of PowerPoint can cause subtle, difficult to fix problems. Best avoided.
Don't save to pre-97/98 versions of PowerPoint
Windows PowerPoint 97, 2000, 2002 and 2003 and Mac PowerPoint 98, 2001, X and 2004 all use the same file format. Earlier versions simply ignore stuff they don't understand in PPT files saved by later versions. Just save your file normally if you want to share it with users of any of these PPT versions.
The rules are different if you use PowerPoint 2007, where you may have to save back to 97-2003 format so that others can use your presentations. In that case, see Staying compatible with earlier versions of PowerPoint to learn more about the effects of "back-saving".
The only other backward-saving options are PowerPoint 95, PowerPoint 4 (in older versions of PowerPoint) and "PowerPoint 97-XXX & 95 Presentation" which saves both current and PowerPoint 95 versions of a presentation in the same file. Result: bloated, oversize files. See Why are my PowerPoint files so big? What can I do about it? for more information.
Unless you absolutely must share files with people who still use PowerPoint 95, there's no reason to save as anything but the normal PPT file type. And for those people, you may find it simpler to give them a copy of one of the free PowerPoint viewers, which will allow them to view and print your presentations in full fidelity. Download Free PowerPoint Viewers
Don't copy and paste pictures and other content from the internet into PowerPoint
Instead, right-click the picture, choose Save Picture As and save it to your hard drive. Then use Insert | Illustrations | Picture in PowerPoint 2007 or Insert | Picture | From File in PowerPoint 2003 to bring the picture in.
If you copy paste from the net, you run the risk of creating a hard-to-remove link to the internet from your presentation. This may cause Windows to try to connect to the internet every time anyone opens the presentation. A real annoyance if ever there was one. And one that can be darned hard to track down and get rid of.
Don't link images
When you insert pictures as described above, you may notice an option to Link to the file. Unless you have a good reason for doing it, don't. PowerPoint's image links break very easily. It's generally safer and more effective to embed the files (in other words, insert them normally, not linked).