Your best tanking wep is a spell power 1h that a mage warlock, shadow priest or elemental shaman would use. It is your primary source of threat.

When you tank, use Ret Aura, Seal of Blood (if not ,70 then rightousness) and blessing on Sanctuary. You'll notice all this stuff is reflective damage - so keep this in mind when tanking. The more you get hit the more threat.

Holy shield on cooldown. It's great threat AND great defense.

Judge Wisdom first when you pull, so you can get some Mana back while tanking. In a multi mob pull you can even just keep seal of wisdom on yourself.

LOS mobs around corners. Don't LOS your healer.

Don't forget to exorcism demons!

If you're tanking and try to cast it stops all your dodge parry and block, so don't do that