Though for some players it goes without saying, the most fundamental and important aspect of being an effective PvP healer is to employ an interface and hotkey system that you're comfortable with. This is paramount, because the healer who has to go searching for that lesser-used cooldown button will not be the healer riding a pro team to the upper brackets. The common view is that clicking is wrong, and in most cases it is, but for every player who has their entire repertoire bound to a pro gaming keyboard, there are others who make things work through a combination of keys and clicking, using addons like QuickHeal.
The rule of thumb for effective healing is to be comfortable with your set-up. You want to make sure that your response time is lightning-quick. You'll only have a tiny window of opportunity to interrupt that Mage who stacks so much haste, his cast bar moves like a cheetah -- but if that comes at the expense of overpopulating your keyboard with binds, consider other options to make yourself more comfortable.