So in the very likely event that a customer clicks on that link, and gets the generic 404 page from their browser, its your designer (or you, if you’re the designer) who gets the credit for losing that customer. Bad bad.

I’m sure you’re not interested in losing customers, not even one. So make sure you have a 404 page that’s not only there, but one that’s helpful too. Here are some tips to make the page better.

• Match the layout with your current site
  Make the page look like any other page on your website, but with a nice little message that tells the user that they’ve encountered a page that doesn’t exist. Match the headings, the text size, everything. Basically we’re going for a page that looks nothing like the big bad error that it is.
• Notify the User of the Error
  This would be a good time to explain that someone screwed up (you may want to put it a little kinder than that). Assure the visitor that this isn’t a common happening. You could even go a little further and tell the user that someone has already been notified about the bad link (which your designer/SEO will see in any server log/analytics report or on Google Webmaster Tools) (also see this script for emailing the webmaster important info)
• Give the User Some Options
  Don’t just tell the user the page is gone, give them some fodder to help them figure out what you screwed up.
  • If you’re a good coder, you can perhaps take apart the bad URL and suck some keywords out of it, do a search for related pages and list those for the visitor
  • Give them a nudge to use the search box (which you’ve included on the 404 page)
  • Provide them with a nice sitemap that gives them plenty of options to find what they were looking for in the first place (see Apple’s 404 for a great example)
• Put Your Contact Info on the Page
  Good lord, do I have to tell you this? Assuming its not already in your header, give them a way to contact you. This may be a good thing, as you’ll find out immediately about the broken link, and you’ll also have a warm lead that you can coddle and caress to your hearts delight.