Check for broken links often with a tool like W3C Link Checker. The Link checker will scour your site to make sure that your site is free of broken links.
Regularly review the content on your site to ensure that it is up to date. Also put a last modified date on the bottom of your pages to show how current your information is. If the content on your pages never changes, users will stop visiting your site.
Check to see how your website looks in a variety of browsers, resolutions, and operating systems. Items such as tables and colors render differently depending on those factors. You can also attempt to print pages out from your website to see how they come out. The same factors that affect how they display in a browser also affect how the content prints out.
Never use the phrase "Under Construction", the dynamic nature of websites infers that pages are under construction at all times.
If you have a very large site, it might be helpful to have a site map, a search feature, or an index page. This helps users find the information they are looking for without having to spend an excessive amount of time on your site.
Be sure to have a feedback link where customers can tell you what they like and what they don't. This will be helpful in finding out what your customers need, and they might have some good suggestions on how to improve your site.