Navigation is very important for both the users and the search engines, follow the below tips to make your website easy to navigate.
Plan the site navigation with the home page as base.
Use breadcrumb lists which helps the users to nagivate back to previous section or the root page easily. You can find more about breadcrumb lists in Google.
Allow removing parts of URLs. Users may try going to upper directory by removing the last part of your URL. For instance, if a user remove the cards.htm part of the URL www.domain.com/2010/cards.htm an upper directory should be available for the user at www.domain.com/2010 instead of 404 error.
Create seperate sitemaps for users and search engines. You can create sitemaps for search engines easily using the open source software of Google: http://code.google.com/p/googlesitemapgenerator.
Use text instead of numbers for navigation.
Have a useful 404 page (page not found error).