Just for personal future reference, but feel free to use if it helps you too!
Using the .htaccess file, you can force www or non-www URLs using the following snippets that should appear ABOVE any other code in your WordPress .htaccess.
Forcing www
Open up the .htaccess file in the root directory of your WordPress install and add the following to the top of the file. (Make sure you replace example.com with your domain.)
RewriteEngine On
# Redirect non-www urls to www
RewriteCond %{HTTP_HOST} ^example.com
RewriteRule ^(.*)$ http://www.example.com/$1 [R=301,L]
Forcing non-www
Like the other snippet, open up the .htaccess and paste this snippet above everything else. (Make sure you replace example.com with your domain.)
RewriteEngine On
# Redirect www urls to non-www
RewriteCond %{HTTP_HOST} ^www.example.com
RewriteRule ^(.*)$ http://example.com/$1 [R=301,L]
