It works like this: I add a brief sentence at the bottom of every personal email I send to clients, but directly ABOVE my signature: "While you're waiting for the next game, here's something of mine that will interest you: http://www....com"
This URL points to a product that is not shown anywhere else, so I know when it appears the sale came from only that source.
By adding this sentence to the dozen or so emails I answer personally each day, I get a sale a day from it. That's almost a 10% return. Over a year the tiny $40-a-day sentence above gives me or my affiliates an extra $7,000 to $14,000. Every little counts!
The secret is to include the offer in the email BODY so that it gets strategically separated from my list of products under the signature.
The main reason for this strategy's success is that the offer comes in an email directly and personally from me - rather than being seen as a mass promotion when it comes bundled in an automated mailing.