The first rule of PVP healing is...you can't heal everybody.
The second rule of PVP healing is...you can't heal when you're dead (unless Holy Priest).
The third rule of ANY healing is...you are not the grantor of immortality; people die.
After learning those three rules, you can now start healing.
Heal yourself first and anybody else that you can; blowing alot of heals on a single pvp friendly is costly, since several other players could benefit from your attention.
PVP is situational healing...there is no right or wrong way...as long as you're healing. I think of PVP as a raid...the enemy has selected a target..that target has pvp aggro..keep them up if you can.
My opinion of course, many other more skilled people may differ