Another solution for using a dedicated healer in a 5 man team is to have separate buttons for healing each character, which is a nice solution for those running without the mouse broadcasting of ISBoxer.

For example in my setup, my healer casts heals on the following targets when i hit the corresponding buttons (those buttons are empty on the other characters, generally):

3: Heal the Paladin tank
4: Heal the Boomkin
5: Heal the Shaman
6: Heal the Mage
7: Heal the Priest healer

The order of the characters is just the order in which my paladin sees them in the party frames.