The best thing to do when launching a new site is use 301 redirects. These tell Google and other search engines that a page has moved and has a new address. That way, all the search engines can update their databases and replace old URLs with new ones, helping to keep any optimization you might have.
One added benefit is that anyone who has bookmarked or linked to an old page will be automatically sent to the new page instead of an error page or your homepage.
I also suggest that use submit a new sitemap to Google (if you use WordPress, there is a great plug-in to automatically do it). Your other search optimization techniques should also be used such as page optimization, titles and most importantly link-building, but start a new site off right with 301 redirects.