Whatever the reason you're looking for a Priest leveling guide, taking a character from level 1-80 can be a long and tedious process. Trust me, I've been there a few times. I used to play Alliance but when my guild fell apart, my brother asked if I would join his Horde Guild and level up a Priest to heal for their 10 man group. Everyone else was at level 80 all ready, and I knew I had to get there as quick as possible.
I looked around online for an awesome Priest leveling guide, and found quite a few that seemed promising. Unfortunately, these guides didn't help me level any quicker than I was going, and most were extremely out of date. You know as well as I do that World of Warcraft updates frequently and some times those guides just don't do a good job of keeping on top of every change each time a new patch is released. Add in the fact that I had to keep Alt+tabbing in and out of game to find out where to go next, it seemed like I was wasting more and more time. I also knew that I didn't want to grind my way to 80 as that is pretty well the most boring way to level, ever.
That's when I discovered Zygor Guides.
The key in power leveling without having to pay for an illegal service, is called "quest stacking". This is the process where you grab a bunch of quests and take them all out at once before handing them in. So much time is wasted while leveing just simply running back and forth between quest objectives and quest givers.
Zygor's Guide tells you exactly which quests are worth doing (some give no experience) while also guiding you on which ones you can take out together, maximizing the amount of xp you earn in a session. It's all in game, meaning no alt+tabbing or playing in Window mode, with an arrow guiding you to each destination you need to go to.
Not only is it important to know which quests you should be doing at what level, in what area, and how to maximize your xp during leveling sessions it's also important to understand what spec you should be leveling with, and where to put your talent points after each level. After using the suggestions in Zygor's guide for your priest, you will be able to kill much faster, and not have to spend so much time eating or drinking to replenish health and mana.
As far as Priest leveling guides go, this one, I feel, is the best.
If you're still unsure...watch as I use the leveling Alliance Guide and put it to the ultimate test.