This plugin allows you to make a WordPress page or post link to a URL of your choosing, instead of its WordPress page or post URL.
more info: http://newplugins.us/wp-redirect-301-or-302/
It also will redirect people who go to the old (or "normal") URL to the new one you've chosen (301 Moved Permanently redirects are standard, but you can choose a 302 Moved Temporarily redirect if you wish).
more info: http://newplugins.us/wp-redirect-301-or-302/