Originally Posted by
Krixooks
Start playing your spriest as though you just hit 90 again.
Go through your spellbook and talents with a fine-toothed comb, and read them, try to work out how abilities that you wouldn't normally use in PvE could be useful in PvP. For example, in PvE you wouldn't use Psychic Horror often, but it could be used in PvP for a number of different things, keeping distance from melee, disarming melee, interrupting spells etc.
Next, you want to keybind everything and by everything, I mean everything. Your keybinds will keep changing, and you'll have to get used to it, but you can't get better at PvP without good keybinds.
For example, abilities that you need to use quickly, like Psychic Horror or Silence, need to be somewhere you can reach quickly when someone is casting a spell at you.
Duel a lot, do a lot of battlegrounds, do 2v2 with good players (for as long as they can put up with you).
Humble yourself, ask people questions. If you don't understand what someone just did to you, ask them to link the ability so you can read it.
Don't switch specs/classes whenever you "get stuck", play one spec/class and learn it inside out, how to react to every situation.