Avoid these mistakes and your website will be steps ahead of your competition:
Failing to put contact information in a plainly seen location.
This could be disastrous. If a customer doesn't see this information, they can't contact you. You should consider a 'Contact Us' button or link from your Home page. Even better, make a link to your email address in your header or footer, somewhere that will show up on every page. Even if no one ever contacts you this way, just the presence of this information comforts edgy customers.
Do you enjoy clicking on a search result only to get a Page Not Found Error? No one likes them. Check your site statistics at least once a month (if not more) to make sure you don't have bad or broken links.
A sure turn-off to a potential customer is the presence of old information. If it's July and your website is announcing the 'new' products available in February, your site just lost major credibility. Make sure your information is up-to-date. Consider adding a 'Whats New' button or a Business Blog.
Too Many Font Styles and Colours
Too many fonts and colours looks bad, unorganized and unappealing. Different colours may attract the eye for a short time, but constant flashing or otherwise bright fonts (and graphics!) become annoying. Beware, this is a sure-fire way to scare people away from your site!
Every website has a hierarchy, a sort of tree that branches out from the Home Page. While most of your visitors visit you through your home page, there are times when a page further down interests someone, and they may copy that link and send it to a friend. This is where you need to pay attention. That friend may like what you have to offer, but they can't find out how to contact you, or how to get back to your Home Page. That's an orphan page. Every page on your site should, at a minimum, have a link back to your Home page. I would suggest adding a contact link at minimum.
Frames at one time were the talk of the industry. They were the original Content Management System (CMS) for your site. Nowadays they are few and far between. If you are designing a site, don't use frames. Newer technologies such as server-side includes are much more common and accepted. Your pages look fresher and those silly bars don't get in the way.
Disabling the BACK button and excessive Pop-Ups
Have you been to a website and decided that it wasn't the information you were looking for? When you clicked the BACK button, did you suddenly get a barrage of windows (or, pop-ups) to your dismay? These things rarely actually work, and worse off, the reason you hit the BACK button is because you DIDN'T want any more information from that site. Don't break the BACK button. There are other ways to get your user's attention.
Slow loading pages
While personal and hobby sites may normally be slow, there should be no reason for your business or other professional website to be slow loading. Today's Internet surfer won't wait long for information from your site - there are too many others with the same thing! Make sure your pages load quickly. If the server is slow, consider a different host. If your webpages are full of applets or large graphics, consider a page/site redesign.
Using Leading-Edge Technology
While the Internet is all about new and fancy stuff, don't be the first to do it. While it may 'look cool' to you, you ultimately need to decide if it actually enhances your user's experience. Do the flashy cartoons make your customer more apt to buy from you? Probably not. How many of your customers have to install a Plug-In just to see your page right? Do they have to upgrade their browser to contact you? Not good. Wait until the technology is either more of a standard or gone - you'll save face with potential and future customers