I just edit the world how I want, place down any families or sims in the houses and do a "Save As" and name the save something like "Blank Save" or something like that, so I recognize it in game.
You should be able to see which "save" it is in your My Documents because of the date on it.
Just make a back up copy of it if you want to keep it somewhere safe. (Just remember that if you make any changes to it and don't back up that new save, if you would ever need to use that back up save again, those changes you made wouldn't be reflected on the original save you backed up.)
Hope that wasn't confusing and I explained what I mean well enough.
