Don't make your file sizes too big

The longer it takes a page to load the less people will go to those pages. On the internet peoples attention span is calculated in seconds not minutes. Make sure all pages including images are under 50K unless absolutely necessary.

Watch the use of background images

If you have a busy background it takes away from the content and the purpose of the page itself. Unless the background is the purpose use shuttle background images and colors.

Watch your color schemes

Even though yellow and black are said to be the most opposite in contrast I have yet to see a site that looks good with a black background and yellow lettering. In my opinion try to stick with what works, black on white(or light grey). You can play with this but look at other sites and see what they are doing. All in all make sure the content is legible.

Use of animation

I see so many sites using extravagant animations, all this does is take away for the content of the page. If you use an animation keep it short and have it stop after the first time it runs.

Don't use the latest new browser features

That browser may have a key feature but if it is the only browser that has it a good portion of you viewers won't be able to use it. Normally new features can be quite glitchy which can turns customers off your site.

Watch the height of your pages

90% of pages don't get scrolled so if you do have to have a long page use inline links at the top so that it is easier for your customers to view what they are looking for.

Make it easy to find anything

Try to make it possible for your viewers to get anywhere in your site in 3 clicks or less.

Most web surfers are normal people that aren't gurus on the internet. You need to make sure that you point them where they want to go. If you have 50 links on the front page they won't know where to start. Keep it simple for them, point them where to go.

Put your contact information everywhere

Sometimes the most annoying part of a site is trying to find out how to get a hold of them. if you want to loose customer this is a quick way to do it, so don't. Make sure your customers can get hold of you.