In my experience wth having red hair, the red will still fade out a bit even if you use special shampoos and conditioners. There's a few things you can do to help slow it down.
1. Limit hair product use. The more product build-up the more you color will fade out when you wash.
2. Try to avoid using clear, clarifying and dandruff shampoos. These WILL strip your color. Moisturizing shampoos are a bit better.
3.When shampooing, wet your hair with lukewarm water and only apply a quarter-sized amount to your scalp only. Massage through for about a minute and rinse by having the water run from you scalp through to your ends. The shampoo being rinsed from your scalp will wash the length and ends.
4. Mix some semi-permenant hair color ( a rinse like adore, jazzing, Punky Colour, special fx, directions, etc) in with your conditioner to keep your color fresh. Apply after shampooing and allow to sit for the time needed and rinse with cool water.