Listed below are some useful and rather important tips for designing a professional, high quality web site:

Easy Site Navigation:
Navigation of links on your site plays a big role in determining the stickiness of your site (how long your visitor stays and explores your site). Try to have a consistent design that matches a visitor's expectations. You don't need to redesign the wheel. Just make it more attractive with less friction.

Clean Layout Design:
A clean layout that uses a lot of white space enhances a site's looks. Try to keep the focus on your content. Use fonts that will be available on all computers to prevent your site looking messed up. These fonts are generally Verdana, Arial and Times New Roman. Verdana is the most web readable.

Optimum Load Time:
Make sure your load time is fast.

Minimize Graphics, Flash and scripts:
They hugely increase your file size.

Stick to Simple Designs:
Complicated designs will slow down your pages.

Cross Browser Compatible:
Make sure you check your site for Internet Explorer, Firefox, Opera and Netscape Navigator

Optimize your HTML & script code:
Make sure that your site doesn't have any unwanted tags or unused scripts.

Design for all Screen Resolutions:
A site that is easy-to-use always encourages visitors to stay and read your content. Try to avoid long pages of content so you can minimze scrolling. Designing layouts that fit any screen resolution will provide visitors with a visually compelling web sites.

Optimize all heavy files: As much as possible optimize heavy graphics, Flash files and scripts. See how popular web sites like google and yahoo have pages that are very small in size and thus load in seconds.

Use Text Links:
What you want to do is to avoid graphical buttons or images and make use of HTML as much as possible. Use CSS Styles for text links to make them as attractive as buttons.

Using CSS or DHTML Layers:
Design with CSS instead of tables is much better for faster loading web sites. Plus, CSS designed web sites enable you to move your main content to the top of the page behind the scenes and this works best for search engine optimization.