Prevention:
1. Normalize three times. This will relieve stress in the steel as well as refine the grain.
2. You could try something like marquenching, also know as martempering even though it has nothing to do with tempering. For that combination of steel you will need a tank of oil at about 450 degrees to quench the steel to just above the Ms point. This allows the thicker and thinner parts of the blade to come to the same temperature before being removed from the oil and air quenched. It also brings the temperature of the blade down into the bay on the IT curve and gives you the time to air quench the steel which produces less stress in the steel.
3. As Mike said, grind your bevels after heat treating, though this is not guarantee against warping.
Correction:
1. Straighten right out of the quench. This requires retained heat in the steel so it would be easier to do with marquenching. It can be done with a regular quench but you have to pull the blade out while there is still good heat in it. Either way, you only have seconds to do what needs to be done.
2. Straighten after a tempering cycle while the steel is still hot. This can be done with a mallet or a hammer or counter bending with a vice. Again you have to work quick.
3. Clamp the blade with steel C clamps to a ridged metal backing and then temper the blade. Do at least one tempering cycle beforehand just to be on the safe side. You also might need to shim the blade with something like ma small coin to counter bend the blade.
4. Cold straightening. Minor warping only. It can be done with a mallet or hammer but you can apply more force if you apply the force slowly, as in a vice. Put a rod at the peak of the warp and two other rods to the side of it on the opposite side of the of the blade in a vice. Slowly tighten the vice to counter bend. Remove to check and repeat the process as necessary. Yes, I have broken blades doing this but it has also worked for me.
5. Repeat normalization, straighten, re-heat treat.
Doug