This is more for tanking instances or group quests, than leveling as prot, as I found ret faster and easier.

First, the buffs.

Always have Devotion Aura up.

Always, always(!) have Righteous Fury up.

Blessing of Sanctuary is very useful, but for lower levels (or ret who are tanking), go with Wisdom.

Seal of Corruption is the best threat building seal, but if you are struggling with mana, feel free to switch to Seal of Wisdom (or, again for the ret tanks, use Seal of Righteousness).

At high levels, the use of the 96969 rotation of abilities is the best way to keep threat high. This involves using skills with a 8-9 second cooldown, followed by a 6 second cooldown, and then a 9, and so on.

e.g. Holy Shield, Hammer of the Righteous, Judgement (Wisdom is best), Shield of Righteousness, Consecration.

Using this method, you are using your cooldowns at all times, and you are maximising threat.

If you tank as ret, just use your normal attacking rotation, but make sure Righteous Fury is up.

On a side note, if you run with a pally and they are not tanking, make sure they don't have Righteous Fury, as this will mess the group up, royally.

http://www.wowhead.com/?talent#sZV0tAMuMusIufdts0hbc:Gpczcm

This is the build I use for tanking.

It makes Judgement 9 seconds, aiding the 969 rotation.

Most everything taken is pretty obvious, if you are PvE tanking.

-Sang.