A good website provides easy Navigation, so that the site's visitors can easily find what they are looking for within the website. Do not create a twelve levels deep hierarchy, regardless how large your site is. You lose audience (visitors as well as spiders) with every hop to a large degree. Define logical nodes in few hierarchy levels, and offer cross-links to related topics where it makes sense.
If you can, go for menu bars on the top left side, which should include global navigation elements like site maps, search forms, contact forms, and the root and category index pages as well. Add more navigation bars as the user goes deeper into the site. Provide links to the most important sections at the top and bottom of each page, along with a path to the root where each level is a stand-alone link. Search boxes should use the GET method, predefined searches for popular topics spread as links are useful for users and crawlers as well. Whatever you do, make it simple, consistent, and logical.
Easy navigation implies fast page loads, so dump your buttons and go for text links instead. Considering keyword relevancy, a text link is a stronger vote for the linked page. With an image link, you have the ALT and TITLE attribute stuffed with text explaining what the linked page is all about. With a text link you can use the keywords as anchor text, what counts more because it's visible to surfers before they do a mouseover. The ideal internal link looks like
keyword-phrase
Create themed sitemaps, each with no more than 100 links, which are linked from the root index page and related category pages. Do not rely on navigation by combo boxes, DHTML menus, or other fancy stuff which is pretty useless for most users and totally useless for search engine crawlers. Do not use frames, especially not for navigation.
As a tweak addressing crawlers and savvy users, you can put one or two lines with keywords in small but visible fonts, linked to related site maps, category index pages, and pages deeper in the hierarchy, at the bottom of your pages.
