Redirect www to non-www:RewriteEngine On
RewriteBase /
RewriteCond %{HTTP_HOST} ^www.yourdomain.com [NC]
RewriteRule ^(.*)$ http://yourdomain.com/$1 [L,R=301]
Redirect non-www to www:RewriteEngine On
RewriteBase /
RewriteCond %{HTTP_HOST} ^yourdomain.com [NC]
RewriteRule ^(.*)$ http://www.yourdomain.com/$1 [L,R=301]
Both of these rules send a full search engine friendly 301 HTTP redirect. They also preserve the entire URL (so yoursite.com/redirects to www.yoursite.com/).
Search engines can usually figure out which format is preferred, but since you're showing the same page for www and non-www URLs it can't hurt to be consistent about it.