If you want to redirect http://www.example.com to http://example.com, you can use this:
RewriteCond %{HTTP_HOST} ^www\.example\.com$
RewriteRule ^/?$ "http\:\/\/example\.com\/" [R=301,L]
- If you want to redirect http://example.com to http://www.example.com, you can use this:
RewriteCond %{HTTP_HOST} !^www.example.com$ [NC]
RewriteRule ^(.*)$ http://www.example.com/$1 [L,R=301]
...