How to backup a Mac running Tiger?
You have two main options. Depending on what you want exactly and how much of your current user information/settings you want to migrate. 

Manually

This is the method I prefer. i believe that when doing a clean install the point is to get rid of all the extra junk that is just sitting on your computer. To do this you need to manually backup your stuff data. Just go through all your folders and copy the stuff to an external HD. (Desktop, Documents, Downloads, Movies, Music, Pictures, Public, Sites, Bookmarks, Adress Book, Mail, etc). 
You leave all your settings/preferences behind. This makes for a real clean install. 

Using backup utility

I would suggest using CarbonCopyCloner. It's free and works on Tiger. With this clone your current hard drive and after doing a clean install Lion will ask you if you want to import your stuff from a previous installation and you can select the clone you just made. Keep in mind that this will also copy back all the preferences/settings and junk you might have accumulated. Thus in my perspective defeating the point of a clean install.

Combination

This is probably the best option. Make a clone with CCC and then instead of having Lion automatically bring back your data, do it yourself manually. This way you will have everything saved from the clone and you will be able to bring back only the important data you want and not any overhead.