Try wrapping your brain around this: short-term permission.
As marketers, we are so anxious to build our lists that we grab every opt-in we can and hold onto it forever. But what if we tried something new? Some products clearly have short-term buying cycles. Think about mortgages, car buying, or insurance. You usually only buy these once every few years. Why do marketers force opt-ins that last forever?
What would happen if the car site's opt-in clearly stated that it would only last for three months? I would love to see whether the opt-in rate would rise -- I'm betting it would. First, prospects wouldn't have to endure mailings long after they've made a purchase. Second, the mere fact of saying that it's short term is likely to add a huge degree of credibility and show customer-centric thinking. I suspect many people would opt-in just to see what would happen.
Some well-designed control group testing should show whether this would work. If any of you have ever tried this, or decide to give this a try, please let me know how it works out.
P.S. For good measure, add in the opportunity for a permanent opt-in on your last few messages to see if you can capture those buyers who have a longer-term interest.