Content freshness is important to search engines. Content Freshness is the amount of content change or new content added. Although you obviously will not want to update your whole site continually you need to plan for both long-term and short-term SEO.
Develop a strategy for short-term SEO where you continually create fresh content
Create a blog with news about your company and related industry
Create a forum for feedback from your users
Include some new pages periodically
Create new links to your older pages
Revisit at least some of your more important older pages and include some more up-to-date content