iWeb stores all your sites in a file called Domain, located in your user's Library » Application Support » iWeb folder. First thing: Pick a location for all your iWeb projects to reside. For this example, I chose the Sites folder in my user's folder. Drag the file Domain to that location. Double-click on Domain, and you should be in business. When you want to start a new web site, rename Domain to any name. Launch iWeb. It will come up with the 'Choose an iWeb Domain file' dialog. Click on Create Domain, and you will have the Save dialog box pop up. Choose the location of your web site, i.e. ~/Sites in this example.
iWeb will launch with a fresh, new project. Now comes the tricky part. Any time you want to edit a different web site, you need to rename that site as Domain. When you launch iWeb and don't have a file named Domain, iWeb will ask you to 'Choose an iWeb Domain file.' If you select Choose Domain as the option, whatever Domain file you select will be the default from that point on. So don't pick Choose Domain; always pick Create Domain instead.
So, why is this knowledge so important? Say you're at your buddy's house or the Apple Store, and you decide you want to create a quick web site. Now you can do so in iWeb, zip it up, and then take it home with you. Delete your web site, and put that Mac back the way it was. By the way, you can work in your user's Library » Application Support » iWeb location for quick new sites. I don't do so on my Mac, because it's a pain to get there and I'm lazy. Enjoy iWeb the way it should be...

