First, you need a shampoo with mild surfactants. Try to avoid sulfates. That's step #1.

Second step is to keep it out of UV rays. Cover up in the sun, or as Dean said use a product with sunscreen.

If you never had to expose the piece to the sun or shampoo, it wouldn't lose much color, but unfortunately that isn't practical, so if you do get the brassy/red/orange tones, you can counteract them with color infusing conditioner. ADF works well, and I also have another leave-in type of colorant that you simply wet the hair with that is great for touchups.

I'm lucky since I wear full caps and don't have to worry about color match. I look at my oldest units and they are BLONDE compared to my newer ones which are more light brown. They are about 4-5 shades different! I need to maintain the color a little more cause it's pretty obvious when I get a new piece if I let it fade out......