This is a tip I posted on the old site, but I'm still so impressed with it, I'm posting it again.

When you come in from the garden with dirty hands and fingernails, wash and brush away most of it with soap and water.  Fingernails still stained?  Grab an old toothbrush, and a tube of toothpaste, and get to scrubbing under your nails.  It does a great job getting the stain out.  A whitener toothpaste does even better! 

maw