My current mistake is having two sites, but wanting to share content across both (they are on the same server.) This was definitely not something that I would have considered when I created my first site on Blogger.
Tom, I really like your point about file structures and content. I am not sure I really thought about that side of things and only do so now that it is a problem. A part of me thinks that it would have been nice to know more, but then again, how could I. I only know what I (think I) know because of the mistakes I made.