49 Shadow is awesome, definitely my favorite twink to play right now.
I'm currently leveling my 49 shadow priest, is the postmaster set the way to go with princess dagger and ramstein trinket? Also what secondary trinket should I get?
That stuff's all worth using, but isn't a huge deal. I've been playing without any of it, just farmed the postmaster set yesterday and haven't gotten around to working on the dagger yet. (Ran strat ~25 times without getting Rammsteins to drop before the patch, haven't felt the need to farm it after getting games in. That trinket makes me feel gross though lol, I don't use it on the character that has it either).
Mara dagger doesn't have any extra stats over heirlooms or other options at 49. BiS, but not a huge difference-maker.
Postmaster set is really only good for the armor/movement speed. My stats are slightly better in my balanced gear set without it, but it's a lot of armor and free movement speed is always good. It'll probably be my go-to when I can't get away with a full glass cannon set now.
You're going to want to switch rams off when its on cd, in which case I'd run the trinket from the pvp crate and either the high chieftain or vigilance charm.
Yeah, I'd say BG trinket (impatient or aurora ideally) > Vigilance Charm >= (new) High Chieftain's = Mindtap Talisman. There isn't a huge difference between the Dire Maul trinkets and a scaled down High Chieftain's. I'd use a Vigilance Charm if I had it, but they're close enough that for now I'd rather save the headache of farming it in legacy loot mode (or LFD). That boss has wayyyy too much on its loot table.
http://www.wowhead.com/item=62966/emissarys-watch and
http://www.wowhead.com/item=21769/figurine-ruby-serpent are also good options for on-use trinkets (they don't share a CD).