how do I prevent printer ink printed on fabric from fading?

This is easy . . . heat set it with an iron, after you peel off the freezer paper, of course. Or throw it in a dryer on high for about half an hour. I've been printing on fabric for a while now, for embroidery projects, using the prepackaged fabric on paper stuff, which is always white. I haven't tried this technique yet, but I know that heat setting works pretty well with ink jets with the other stuff. Just don't wash the bejeezus out of it. Should be fine for low use stuff.

First thought - use decent ink. The "proper" inks for printers are more light-fast than cheap refills. Keep it out of direct sunlight. If it's on a garment, wash by hand, dry gently.