1. Trust Text
It's tempting to spice up pages with graphics - but sometimes even a little is too much. If possible your navigation system should be based on text links, rather than image maps or graphical buttons. Studies have shown that visitors will look at and try text links before clicking on graphical buttons.
2. Next Best ALTernative
If you must use a graphical navigation system, include descriptive ALT text captions. The ALT text will make it possible for visitors who use text browsers such as Lynx or who browse with graphics turned off, to find their way around. In addition to the graphical navigation buttons, be sure to include text links at the bottom of every page that provide a clear route to the main areas of your site.
3. Map It
A site map offers a good overview of your site and will provide additional orientation for visitors. It should be in outline form and include all the major sections of your site with key subpages listed beneath those sections. For example, you may group your FAQ, Contact and Troubleshooting pages so they're all accessible from a Support page. It's a good idea to visit a few larger sites to get some ideas on designing an effective site map.
4. Forego Frames
Avoid frames wherever possible. Most veteran browsers dislike them and they can be confusing for visitors who are suddenly presented with multiple scrollbars. If you're committed to using frames on your site, you'd better commit yourself to some extra work too, because you'll have to create a no-frames version of your site for visitors whose browsers don't support frames.
5. Consistency Counts
Don't change the location of your navigation elements, or the color of visited and not-visited links from page to page. And don't get clever with links and buttons that appear and disappear: turning things on and off is usually done as an attempt to let visitors know where they are at a site but more often than not it ends up confusing them.
6. Just a Click Away
Keep content close at hand. Every page on your site should be accessible from every other one within four clicks. You should regularly re-examine your page structure and links, and make necessary adjustments. People come to your site to find information - don't make them dig for it.
7. Shun Search
Most sites have a search function, but try to discourage its use as much as possible. Even the best search engines turn up irrelevant matches, and visitors may not know how to use yours effectively. Logical, clearly placed links are more likely to help visitors find what they want.
8. Passing Lanes
Provide multiple paths through your site so visitors aren't restricted to one style of browsing. For most sites, a pull-down navigation menu is an easy addition that offers an alternative route through your pages, without wasting space.
9. Overwhelming Options
Don't overwhelm visitors by presenting dozens of places that they can go. A large number of choices is not necessarily a good thing.
Finally, if you feel like curling up with a good book, I recommend Jennifer Fleming's Web Navigation: Designing the User Experience from O'Reilly & Associates