I originally posted this on the SW:TOR boards here. Given the positive reception, I'll post it here too.
1. You shall initiate the fight when appropriate.
As the tank you are the most likely of your team to absorb the initial focus of the enemy team and live, so it's your job to pick the fights. You will go in first and start messing with people for a couple seconds before your team engages. The idea is that the enemy starts with focus on you and forces the enemy to interrupt a damage rotation and take a second to switch targets to start hitting your big DPS character. The seconds you buy from this can make the difference between winning and losing the fight.
2. Defense before offense.
Your primary responsibility as a tank is to keep your DPS alive so they can kill the opponent. Adding assist damage to your DPS's target, while very important, is secondary to your other functions such as Taunt, Guard, and various CC and damage debuffs. Your DPS can do no damage dead.
3. You must be a threat.
You don't necessary need to make an enemy go, "He's gonna kill me, get him off." It helps if you can do that while still being able to initiate, but you just have to be enough of a pain in the rear to the enemy to make them deal with you. It can be with CC frustrating the enemy to the point of focusing you just so they feel like they can act. It can be making the enemy's primary healer or DPS useless by debuffing them. One of my favorite things to do to a sorceress in WAR was to start by dropping her damage to 70% of normal, cutting down her crit chance and draining her morale, then disorienting her to make all her casts take 50% longer, followed by a snare, a silence, then draining her AP and morale further. It usually got me some attention.
4. Take advantage of player psychology.
This really applies to everyone, but the PvP tank can do this in ways no one else can. In WAR, I always carried 2 full sets of gear and had a few preset combinations of them I could switch to with the push of a button ranging from balls to the wall offense to a couple balanced sets to full turtle tank. One thing I would often do is try to project an image of being one end of the spectrum and then switch to the other. If I wanted to be a full tank for example, I'd rush in, lay some heavy damage on a enemy, get focused, die, respawn or get a rez, switch gear to full defensive gear, and then go in like that for the rest of the fight. It also works the other way too if I want to DPS. This is similar to a poker player presenting a table image then switching to another play style hoping his opponents don't notice.
Of course, you want to communicate something like this to your team ahead of time if you can. Going in first as a DPS to give an early, easy kill can also make the enemy overconfident, and you definitely want to take advantage of that if you notice by luring the enemy into traps.
5. The life of your DPS is more important than yours. (But not by a whole lot)
You should be willing to sacrifice yourself to save the life of your DPS. Your DPS should NEVER die before you do. Obviously, this does not always happen, but it's the mindset to have. If your DPS is going down before you consistently (and it's not his fault for being stupid), you need to make yourself more of a threat by shifting some of your survivability for damage. This works the other way as well. If you die well before your DPS consistently (and it's not your fault for being stupid), you need to trade damage for survivability. Ideally when your DPS dies, you should have been killed seconds earlier or be about to go down shortly after the DPS dies. In this way, you'll have a perfect balance of threat to survivability. This is presuming the DPS player is competent of course. If your DPS teammate couldn't DPS his way out of a wet paper bag, he's not worth your life.
Guard damage should be one of the major causes of your death when it happens. Yes, you can take more damage than most, but that's because you have to be able to do so. You intentionally redirect some of the incoming damage from a teammate onto yourself. You are able to defend and mitigate this damage. You should be able handle this damage without being a major drain on the healing resources which are better conserved to keep your DPS alive.
6. If your DPS is wrong, you should be wrong with him.
This is related to number 5. This means if the DPS you are protecting has run off by himself to do something, you need to be with him. Your DPS is useless dead, and a lot of your abilities to mitigate damage do not work on you, so you might as well be useful with your DPS being wrong, than useless being "right."
Taunt for example, unless it changed from the Eternity Vault video, reduces damage of the targeted enemy to everyone else except you for a time. You can't Guard yourself, not that it would do anything dividing the damage you take between you and you.
7. Tunnel vision will kill your team.
You need watch health bars like you were a healer so you can place Guard where it is needed. You need to watch the enemy so you can place your CC, buffs and debuffs effectively. You need to watch the positioning of your team since Guard only works within a short range. (See also number 6 for this.) You need to do all this on top off watching yourself, your ability rotation/priority, your health and resources, and your positioning. It can be very stressful, and sometimes impossible, to keep track of all this.
Sometimes in WAR my healers would tell me to potion because their resources are drained or focused on another player. They did this because they understood sometimes I can lose track of my own health trying to perform all my other duties. I rarely needed the reminder, but I appreciated it.
The PvP tank is a thankless job, much like the PvE tank, but it is very satisfying when played well.