The best way is to use pigment-based dyes in the printer, as these dyes are water insoluble. If your printer uses ink-based dyes, there is no way to make them absolutely permanent. Some say that heat-setting helps.
I read recently where one quilter says she irons wax paper on the label and then lifts it off. Apparently the wax helps preserve the ink. If you do this, I would do it on both sides of the label. Also, be aware that you need to use parchment paper between the wax paper and your iron; otherwise the paper can stick to your iron and burn there.
Did you use regular fabric to print the label? Or is it one of the treated fabrics?
Edit: Here is a link to an article that explains why pigment-based dyes work better:
http://www.gloriahansen.com/weblog/?p=6060