Yet Another Warlock Nerf has a terrific guide up to one of the most misused/underused pieces of gear in the game -- the PvP trinket. The cheapest version, Insignia of the Alliance/Horde is just over 2,500 Honor, so anyone can get a version of this (YAWN recommends the Rare version, which is powerful enough to be useful and cheap enough to spend honor on other things) and especially until many more specialized trinkets are available at endgame, it's a must have for anyone running PvP.
But when to use it is the question. Especially in 1v1, the best times to break out your trinket are when it will allow you to immediately avoid damage, so breaking it out for things like Sap or Polymorph (which don't actually cause damage to you) can be a waste of the cooldown. But things like Kidney Shot, Fear, and Frost Nova mean that more damage is incoming, so then is usually (there are, of course, exceptions) a good time to pop the trinket and escape the damage.
YAWN bravely goes out on a limb here and tries to provide some guidance for an activity that isn't very well governed by guidance (don't tell PvPers what they can't do!). Actions and rules are very, very situational, and so there sometimes will be times when you should trinket out of Sap, or spend your two minute cooldown on Death Coil. But as a starter guide to when to use and not use the trinket, it's a great read.