How to do I keep my dark hair color from fading so fast?

Paul Mitchell has a shampoo for color treated hair.

Use Ion Effective Car Intesive Therapy Conditioner (it comes in much larger sizes than this)
http://www.sallybeauty.com/hair-repair-therapy/ION22,default,pd.html

Use higher quality dye – makes all the difference in the world.

Dry shampoo, available at pretty much any drug store for around $5 – $7 :) That way you wont have to use regular shampoos so much, which will fade it faster.

Oh. I was going to say, “Don’t wash your hair as often,” but that is absolutely unhelpful. What if you wear a hat once in a while? Let your hair build up some oil for a day or so? Is there a way to prevent your hair from getting oily? That’d solve two problems at once!

Shampoos like head and shoulder will strip the dye out. Next time you dye, use Gray Magic. They are drops you can add to your dye and it will make it last longer. You can purchase it at Sallys.
Also be sure to use the conditioner as told in the directions and wash and rinse with cool water. Many people either don’t use the conditioner that comes with the dye or leave it on a short while. Many of those conditioners have properties to help the dye adhere to the hair but you have to leave it on the length of time suggested.

L’Oreal dye and shampoo- no problem. Dry shampoo?? Emerg only.

I use Burt’s Bees shampoo for color treated hair. With my short hair, I can use a smidgen of the conditioner that comes with the dye every time I shampoo.