Even before I optimize websites I like to get a good deal of new content down in order to insure that I know exactly where I’m going and exactly what I need to do to get there. Creating some of the new content before starting the optimization process can be doubly helpful in that it can reveal potential additions to your website that you may not have considered (a forum or blog for example).
If you already have a site, perhaps simply sit on your back deck, sip on a coffee and image what you would do if your whole site was lost and you had to start again (other than launch into a very colorful discussion with your hosting company).
A solid site structure is very important. Creating a site that is easily spidered by the search engines yet attractive to visitors can be a daunting and yet entirely rewarding endeavor. To adequately structure your website you must "think like a spider" which is not as difficult as it may sound. A search engine spider reads your web page like you would read a book. It starts at the top left, reads across, and then moves down.
Priority must be given then, to what you place near the top of your page.