What's the best way to do a clean install of Mac OS X?

If you are a .Mac member then you have access to Apple's "Backup" utility. (you may also use Carbon copy or any other backup programs out there)

I did a full backup to DVD for my iTunes and my iPhoto library.
(The cool thing about backup is that even though the resulting files on the DVD are "supposedly" compressed, you can right click on it and navigate down into the folders and pull the songs or images you want and leave the rest intact.)

Backup your Documents, Movies, images (pictures) folders

If you use iWeb then you may want to backup your website files at:
/Library/Application Support/iWeb/
(You can toss these into the same folder once you are in Leopard)

Mail, iCal and iWorks also have folders with all your e-mails, calendars, files & settings... you can back those up too.

You can find the exact folder location if you do a search at the apple support site:
http://www.apple.com/support/software/

After that you can safely delete your entire HD and install Leopard.

You could use CC and do an entire HD backup but then you would be trying to fetch the correct files from a compressed volume or from an external HD using the same folder locations as before.

One thing about iCal you may want to export a copy of your calendar and save it on DVD just in case. Mine was locked when I copied the files over and it was set in subscription mode. so even though I am able to see it I can't edit my old iCal dates. Now, I just add extra dates and events to my new ical while still having the other one in the side bar.

That's all...

Good Luck ^_^