Generally I agree with what Joseph is saying... "I know this might sound really obvious, but if I had done this instead of trying to cram in demos for everything that I wanted to show I think my presentation would have been a lot better."

The other one I like (which Don gave me) - "Facts obscure the truth.", it was troubling for me at first, but now I really believe it. It is almost always better to omit the minutia about a topic and just talk about the key messages or ideas you want to get across. If you presentation really is about the obscure details of some small topic, great, but most presentations aren't.