Anyway here's mine but hey, you might have already thought about it
Most of my customers use my email services for business use, so a perfect way for me to reach a targeted market for my services AND where you will get a glowing testimonial from your existing customer, is using their outgoing signatures. Obviously not everyone is going to want a 3rd party sig at the bottom of their business emails, but for incentives, many would! So by offering a small discount off their renewal, or offering additional accounts or space etc (whatever they will go for!), most will come around to the idea of having a useful, targeted message at the bottom of their outgoing emails.
So what I've done using the very useful AMS Content Filtering is add the agreeing customer's individual email address or domain name as a condition, and Add Signature as an action with a witty, catching message and whoila! That customer is now marketing your business to every other contact or business they deal with on a daily basis. You don't get better marketing than that!
If you got any useful tips, feel free to post them!