What should go into your custom 404 File Not Found page? It is insufficient to simply let the visitor know that the file could not be found. In order not to lose that visitor, you will have to provide him some way to locate the document he wanted, or you would have lost him.
Web Design Services page should have one or more of the following things: A link to your main page, with a suggestion that the visitor can find what he wants there. If you have a search engine for your website, you should definitely put a search box on that page. Many people prefer to simply type a query than to scan through your site map.
A link to your site map, which lists all the pages on your website. If you know of frequently mistyped URLs on your site, you can even put links to the correct location directly on the page, so that visitors who arrive there from outside can quickly get to the correct page. Remember, you don't want to lose that visitor, so do all you can to help him.
Any other navigational aids that you may have - for example, if you have a drop down navigation menu on your normal pages, you should probably put one here as well.
If you like, you can even put a simple form on the page to allow your visitors to inform you of the broken link. However, the primary aim of this page is not to help you track bad links, but to make sure your visitor does not leave your site if what he wants can be found there.
Incidentally, you should make your 404 page larger than 512 bytes, even when you are testing. Otherwise Internet Explorer (IE) will load what it calls its built-in "friendly HTTP error message" instead of your 404 page.