What advices would you provide to update a website properly?

According to your opinion, how often your website should be updated? What website updates are the most important for a website? What advices would you provide for people who are going to update a website?

I think that sitemap is very important. There are two types of sitemaps. The first type of sitemap is a HTML page listing the pages of your site - often by section - and is meant to help users find the information they need. While this type of sitemap can help both visitors and search engines navigate your site, it's different from an XML Sitemap that can be submitted to Google using Webmaster Tools.
XML Sitemaps - usually called Sitemaps, with a capital S - are a way for you to give Google information about your site.

The biggest advice would be to keep it from getting too out dated. Lots of blogs and websites are obviously years old, so as long as you keep it current there really is no set schedule. If you change the style a little bit (maybe new banners every once in a while) that will catch the eye of your visitors and peak their interest. The content is the most important part to update though, so as long as that is fresh it should be good.

This topic is far too abstract. Some websites will never need to be updated; others will require new content daily. There isn't any sort of generic content rule that can cover all types of websites.