It may seem obvious but...

Listing of obvious tanking tips

1 - In General

• Keep what you're tanking in front of you, you cannot parry, dodge, block what is not in front of you. ( I've had to teach people this when I'm playing my alts )
• Get crits off your hit table, this means defense 535 in heroics and 540 in raids.

2 - In a group

• Make sure your group knows what you're hitting first - I put a skull on my main target
• Don't pull until your healer is ready, my healer is usually my wife and she's at the point now where her mana regen lets her be always ready
• Make sure your healer is ready, nothing suck worse than seeing your health go down and down and then die, only to find out your healer when afk to get a drink.
  

3 - In a raid

• Communicate with your other tanks, make sure everyone knows who's doing what.
• Make sure you know your role, if you have a question ask the raid-leader. If you're the raid-leader make sure everyone else knows what they're doing.
• Use your consumables, usually a tanking flask like Flask of Stoneblood or Flask of Fortification if you're still a little low on defense. Use food, if you're lucky or can make it yourself you can get a Fish Feast.
  

I'm sure I can come up with more, but these are some of the things on my mind now.