Only one real thing to do with priests to be successful in PVP: spec Circle of Healing and Unbreakable Will. The rest is gravy.

OK, so I'm employing a bit of silliness but I can't recall the number of times I've been able to survive gank attempts by people 10-15 levels higher than me due to having an instant, uninterruptible AoE heal. At 70 with trash greens, it heals for 1100-1400 a pop, every 2 seconds. Quite a lot more with trinkets popped. For most world PVP, that's plenty. BGs are a different story but that's true for anybody.