[Halfdeck]

no www/non-www redirect installed

If you have people linking to both "http://www.example.com" and "http://example.com", you're splitting your backlinks, because to Google, those are two unique URLs.

For example, if site A links to "example.com" and site B links to "www.example.com", you really only have one back link to example.com and one back link to "www.example.com". On the other hand, if both site A and B link to "www.example.com", then you have two backlinks pointing at "www.example.com".

So if you have links pointing only to one version, you'd end up with a much stronger page.

Same deal with example.com/ and example.com/index.html - if you refer to both URLs you're again splitting link juice and making both of those URLs weaker. My preference is to link internally to example.com/ and not example.com/index.html.

So first, you want to be careful how you link to your own pages.

Second, install a 301 redirect from www to non-www (or vice versa, depending on which version you prefer) so that all links pointing to the www version will be credited to the non-www version.

To do that, you can add a mod_rewrite in your .htaccess (non-www to www):

RewriteEngine On
RewriteCond %{HTTP_HOST} ^example.com
RewriteRule (.*) http://www.example.com/$1 [R=301,L]

(replace example.com with your own URL)

Or (www to non-www)

RewriteEngine On
RewriteCond %{HTTP_HOST} ^example.com
RewriteRule (.*) http://example.com/$1 [R=301,L]

(I prefer the www version, so I haven't tested this)

Use a HTTP header checker to make sure your code is working. You can also use Xenu Linksleuth to make sure your site is crawlable after the 301 install.

Or you can go into Google's Webmaster Tools and set your Preferred domain. That will take care of Google, but you'll still need to install a 301 redirect for MSN/Yahoo.