Make sure each page in your website has something valuable to offer.

Though this doesn't really relate to design, it's actually more important than design, which is why it's the very first tip. I know many people reading this page are trying to find out how to make useless pages look pretty, because they think that style is all that really matters. So let's step back a minute and realize that fundamentally a web page exists to provide something that's useful or interesting to visitors. If your page doesn't have that, then you must fix that problem before you worry about how to present it. If you throw mud at a canvas, then even if it's in a gold frame, it's still just a canvas of mud. What are you offering to your visitors? Why is it worth their time to visit your site? Please focus on that before you move on to how it should look.

If your plan is to make money from advertising, then go for a ratio of not less than 75% editorial to 25% advertising. Amazingly, I see some sites that are almost nothing but ads. We know that no one would turn on the TV if it were just commercials, and no programs, or buy a magazine if it were just ads, and no articles. By the same token, a website also has to have more than ads if it's to be successful.