The webmaster team at Google has published a one page SEO guide that covers three basic but key factors that will help improve the SEO score of your website. These include:

1. Write unique meta descriptions for each page and these should be 160 characters or less.
2. Your images should have an ALT tag describing the image and always include captions below each image (SEO for images).
3. Update often and keep your content up to date (hint: revisit and improve the older content of your website).

Also see: Google Experts discuss SEO