Shadow Priest

First off, I will not give you levelling locations, everyone has their own preferences, this guide is to show you how to manage your Mana and how to efficiently level the priest with as few downtimes as possible.

Bags:

Your first priority should be to get more bag space. You do not want to be running back every 30 minutes to empty your bags. And you will need the gold!

You can get Herb a bag or two, these wonderful bags can only hold herbs but are bigger than other bags. Personally I seldom have more than 10 stacks of herbs in my bag, and when I do I'm usually about to return to a city / inn where I can either put everything in my bank or mail it to a carrier toon. On the other hand if placed in a bank slot a herb bag is advantageous if you are planning on going for Alchemy.

(thanks to Huka for pointing this one out)

Auction House (AH) and Gold:

Auction house is a great way to loose your hard earned cash! Keep in mind that most people will be checking the AH between 18:30 and 20:00. There are also many more people online on a Friday evening than a Monday evening. Knowing this, put your auctions up at about 18:00 for the shortest amount of time (2 hours). It will usually get sold and you don't pay that much on commissions. Yes, the AH takes money for selling your items, the longer an auction is up, the more you pay. If you don't sell an item after having put it up 2-3 times, sell it to the NPC.

Sell anything green in the AH, leave the start bid as it is, put the buyout to about 30% - 50% more than the start price for green items.