Hey there. I just did a brief search around the forums for any sort of comment or post related to this one. I did not find any. If there is already a post for this, I apologize. I'm just trying to contribute.
This is a very simple method to never lose a duel in WotLK as a Death Knight.
(Why is it good? There are Duel achievements and stats in WotLK, so you can achieve those easily.)
First off, you need to spec into "Shadow of Death" in the Unholy tree, which is tier 4. That allows you to resurrect as a ghoul whenever you die. In duels, the game thinks you die, as you get to 1 health. You will become a ghoul with 100% health.
What to do:
Start your duel, go all out on them, get them as low as you can. If they manage to beat you and get you to ghoul form, continue beating on them in that form. Unfortunately, Ghoul form is very squishy and will die rather quickly.
Once you begin running low on health in ghoul, right click the buff on the top left of the screen that turns you into a ghoul to end it. It will return you back to your original form with ~1 health. You'll die in one hit. However, you will turn back into a ghoul with 100% health and 100 energy.
This is repeatable until you win.
You can end ghoul form while stunned. Doing so will break the stun or any other effect that causes loss of control. (Fear, Stun, Slow, Daze, Poly, Hex...)
I've used it to get a 100-0 duel victory with a friend of mine and a fancy achievement. I've also used it on other faction members who /duel me when I'm not flagged. I get them low on health, go to ghoul, end the duel, and beat them in ghoul form. After ghoul form ends, it brings me back to human with 100% health.
This doesn't work in PvP that will actually kill you (I.e. wintergrasp, BGs [NYI]) or PvE. It's just a duel method.
I hope you guys enjoy it