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For my employer's site, I put the basic navigation bar and the site map on our 404 page [absak.com]... so you really can get to everywhere on the site from there. When I look at our logs for 404s, the individual visitor paths most often show someone entering a faulty URL, getting the 404, and then going to the correct page (or another page on the site), rather than leaving. Seems to be doing the trick.
Also it seems that it is more fun to build something new than it is to fix something that doesent work.
Well, IMHO anyway.
I find myself having the same exact problem. It's hard to step aside from my paradigms and schemas and to think and carry out tasks the way other users would.
Jakob Nielsen has written some articles on usability testing and ways of finding out how other users react to your sites depending on their level of experiences. When I find one I'll post it.
LOL... *some* articles? :) I think he wrote ALL the articles on those topics...
And Mr. Nielsen's expertise aside, the best thing you can do for your own sites is to watch your logs like a hawk (IMHO). If a lot of your visitors seems to leave in a certain section of your site, go look at it and ask yourself why. (Unless it's your links page, or the last page of the checkout routine in your shopping cart... ;) )
If you change something on your site and see the average # of pageviews per visitor drop from 5-6 to 1-2... you goofed. Go back and undo the change.
And take the time to build custom error pages for your site... at LEAST build a custom 404 page. Give a link to contact the webmaster, and links to other major areas of your site... make it easy for visitors to get back on track (or email you to complain) when they hit a 404.
Below is a link to a great page dedicated to 404 pages, and he has even more detail on how to set it up on various flavours of webservers.