Coffee stain remover seems to work the best for me on those long-lived stains. I have found it works best if you apply it when the carpet is dry, let it set for 15 min., then flush. If the stain remains, mist a little more on the stain and walk away, explaining to the customer that it may fade away in a week or it may not. I ask the customer to call in a week if it is still there so I can try once more. They usaully don’t call.