Tip 1: Generally for a website do it one bit at a time, staged, rather than overnight. Allows you to keep some rankings in some parts of the site. Ensures clients don't find it too much of a shock.
Tip2: Before you start, rename index.html as index.php and leave it there for a week or two with a 301 redirect. When it comes to switching then at least (perhaps - I'm not a techie) the search engines are already expecting index.php
Tip3: http://performancing.com/wordpress-tips/wordpress-hacks-moving-static-html-site-wordpress
Tip4: http://www.themespress.com/
Tip 5: http://urbangiraffe.com/plugins/redirection/
Tip 6: Keep your existing html site. Parallele to it build your new wordpress site. As in tip 2 above, only have the index.php page of your theme searchable i.e. duplicate the existing html page, rename it index.php and put it in your theme. In wordpress go to the part of the admin panel that says don't index this site. This automatically puts no index, no follow in all your WP pages. You now have an invisible (to the search engines and users) WP blog being built whilst the old html site still sits there being indexed and still visible to users. When you're ready, untick the appropriate box and your WP site will start being indexed. Redesign your index.php page in your theme (i.e add in a loop, sidebar etc.) so that it links to the rest of the blog. You should have been building an .htaccess file on your PC, with redirects, which you can now upload. The plugin from Tip 5 will log all 404 errors so you can track down whatever pages haven't been 301d correctly. What you don't want is too much duplicate content, so why not spend a few months transferring the whole site as above to WP ?
clivemcg
richarduk