I've heard a lot of SEO's say that you can't optimize a Squarespace site as well as you can a Wordpress or a custom site and I most definitely disagree. While Squarespace is a closed platform and you don't have full access to all of the code, even within the developer platform, you still have access to everything that you need to optimize your website.
Squarespace has done a fantastic job of building a solid framework that is built with SEO best practices in mind from the beginning so there are only a few simple guidelines to follow when building your Squarespace site to make sure it is fully optimized.
Squarespace's native features include: Automatic Google Sitemaps, Canonical Tagging, Facebook Open Graph Support, Robots.txt, Clean URLs, Google AuthorRank Support (which Google sent to the dungeons this past year), Page Titles, Automatic Tagging, Automatic Redirects, and Anti-Spam. (http://squarespace.com/feature-index#seo)
Automatic image optimization, progressive loading, etc...
Switching your site over to Squarespace, don't forget to forward your old url's to your new relevant pages. Squarespace has you covered for forwarding url's.
Here at Simple Group we are happy to provide on-site SEO analysis for your Squaresapace site and help you optimize it, help you through your transition of switching to Squarespace as to not lose any of your linking juice from all of your past work.