I have already posted solution before... please search before posting :-)
Also make sure you are not adding www in admin->general->Preferred site URL
For www to non-www
DirectoryIndex index.php
<IfModule mod_rewrite.c>
RewriteEngine On
#RewriteBase /
RewriteCond %{REQUEST_URI} ^(.*)//(.*)$
RewriteRule . %1/%2 [R=301,L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^.*$ index.php?qa-rewrite=$0&%{QUERY_STRING} [L]
# Redirect www to non-www
RewriteCond %{HTTP_HOST} ^www\.(.*) [NC]
RewriteRule ^(.*)$ http://%1/$1 [R=301,L]
</IfModule>