There are some really great ideas here!
One that works for me all the time, gets a high open AND reply rate is one that I use for unsold prospects who I'm having trouble contacting after a while. Funny, I'll call them, no answer. Send the email, two minutes later, they're on their phone opening and responding.
Subject line is "The suspense is killing me"
Body goes like this:
Hello <{TitleCase}><{CustFirstName}><{/TitleCase}>,
I've tried to contact you several times and the suspense is killing me. It's important to me to know that you have received my emails and phone calls. I want to make sure that I am getting you everything you need to make an informed decision, however I don't want to bother you with endless emails and voice messages.
Please simply reply to this email with just one of the following words typed in the subject line to help me understand how or if I should continue to follow up with you:
1) IN (I am still "in the market", I just haven't had time to call or email back. Feel free to contact me ASAP)
2) INFO ( I may be interested if I could get more information and details)
3) DRIVE ( I am interested in taking this vehicle for a Test Drive)
4) OUT (I have decided NOT to buy at this time, so, I'm "out of the market" for now)
5) BOUGHT (Tough luck, I bought a vehicle elsewhere)
6) JUMP (Leave me alone, go jump in a lake, and don't contact me again unless you are giving away cars)
Thanks for your time!!!