Search engines are like gardeners. They like to see things grow. Nothing makes them happy like dropping by a web site and finding more content. The more often they find new content when they drop by the more often they check out your site.
A site that is regularly updated with "quality" content is going to indexed more often and ranked higher than a stale site. While minor updates and content corrections are nice, nothing beats adding pages regularly to your web site to really keep search engines interested. New content is great, but make sure to keep on subject with your content. No search engine is going to penalize your site for the occasional off topic pages and posts, but if you stray too far from your sites subject, you might become less of an authority.
To make updating your site easier I suggest that my clients implement a blog, or content management software. This is a user friendly way for even a non-technical person to edit and post a web page in their site. If you can compose an e-mail, then you can use these. The best part is that most of this software is free and fairly straightforward to integrate in to your existing web site. I personally use the Serendipity blog software and Joomla Content Management software to make publishing client content as well as letting people subscribe easily to updates. If you are looking to make your site more marketable with a blog, or be able to update content easily, feel free to comment here or contact me at sales@cr-sg.com. Pura vida!