I was really annoyed last night in Naxx-25 having to swap back and forth between Prot and Arms.
First, I had to change my dual spec tree over, then swap gear (I use ItemRack) then change my stance. This got tedious after the dozenth time of swapping so I commited myself to finding a way to make this simpler. Today I found it.
Make 2 new macros named ActDPS and ActDef and give them the ? icon.
Give them the following Macro Text
/cast Battle Stance
/cast Defensive Stance
Change the 1/2 to whichever talent spec you have set to that group.
Now, with one click of a button you can change completely from one thing to the other, no sweat. (Note that you cannot change gear or talents in combat)