Khalil Majdalawi's Blog

its very simple SEO trick to avoid duplicate content on Google search engine and increase your rank , you have to chose which one suit you www or non-www

first you have to enable “rewrite mod” which is apache server feature click here

then use the following lines in your .htaccess file
to remove WWW

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

to force WWW

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

Please, note that the .htaccess should be located in the web site main folder.

for farther reading about .htaccess Check this great post