For a good example, look at this guy: Dorkness @ Tichondrius - Community - World of Warcraft
His heirloom shoulders are only ilvl 15, but his helm is 90. This means that even though that helm has no stats it's going to out-perform heirloom items. With a disc priest like the guy linked above, you can hit 800+ penance crits. It's very easy to see games of people going 50/0
So where's the guide? Well, if you look at the items they wear, almost all of them can be bought from vendors. Twinks who don't know where to get these items will pay huge amounts (I've sold two of the rings for 5k each). Here's the ones you'll want to pick up.
Stratholme Lily from Sophie Aaren - 8g50s58c
Problem Solving Pendant from Griftah - 100g
The Rock from Arred (Exodar) Gappy Silvertooth (Azshara) Gelanthis (Silvermoon) Qixdi Goodstitch (Cape of Stranglethorn) - 100g
Mist-Piercing Goggles - crafted by Engineering
Haliscan Pantaloons is probably another BiS, crafted by tailorers