it could be the quality of your dye, and how well your hair takes to dye. do you use a stylist? it can be expensive, but if you've got the money its worth it. the stylist uses the good stuff, and when you go in for a trim you get a glaze to brighten things up.
Use only light shampoo on your hair if you have to and do it only after about 48 hours since coloring. You should wash you hair with color extending shampoos about two times every week. Only cold water is to be used when washing hair since warm or hot water would hasten the speed of color fading. Here are more tips http://www.ehow.com/how_6608497_make-red-hair-color-last.html .