Sure, every farmer knows that rotating crops helps the soil recover and increases yield.
What about email farming? I was talking to Josh today about this and it occurred to me that the rules are the same.
If you're sending emails regularly, don't send the same thing. Don't send the same format, don't send the same offer, don't ask for the same response. Rotating your offers and your interactions maintains interest, surprise and makes your core offers more appealing.