I think timethief offered very good advice and you mentioned that you are also trying to maintain good SEO habits, which - in my opinion - is the most important part.
WordPress itself does a lot for SEO already: a rel=”canonical” link, rel=”prev” / rel=”next” links, an XML sitemap (this is yours), automatically sending notifications to Google with every post and page update, and much more - which really helps along with indexing your site and making it searchable.
Additionally - as timechief mentioned - the vast majority of (free) Themes on WordPress.com are also SEO optimized: microformat support, formatted titles, post/archive navigation, site structure, post title as the first anchor text, some even show a taxonomy description or have a breadcrumb.
So my number one advice on top of that would have been: Write regularly high quality content. But you already do that! :)
So here are two more things that might help you along:
Hope this helps!