Updating your web site is important because it keeps things timely, and your customers will respect the information on your site because they can see that it's getting updated.
However, in addition to frequently changing the homepage and other pertinent pages, plan to go through your web site every three months or so to make a change on every single page. Search engines like to see that changes are made to a web site.
For example, the "Contact Us" page probably doesn't need frequent changes, but every three months, plan to make a small change to that page.
The search engines will smile upon you favorably.