This is just an example of what I would do...

Bind Priest bars to keypad keys as follows:
keypad 7, 8, 9 - renew, flash, greater heal mage 1
keypad 4, 5, 6 - renew, flash, greater heal mage 2
keypad 1, 2, 3 - renew, flash, greater heal mage 3
keypad 0, . (period), enter - renew, flash, greater heal pally

You get the idea. You can make the keys/bars anything you want really.

Use macros like:
#showtooltip
/cast [target=mage1] Flash Heal
/cast [target=pally] Renew

You could add some modifiers as well to save some space, although it may be hard to reach unless you do a left-alt + keypad.

I use the keypad because most of my normal keys are used on all toons for something.