Dan, some ILM's think there is a secret sauce to this...and hey maybe there is but I think it's deeper than the follow-up schedule. You can have a stellar follow-up schedule but without the "right message" what good is it?
There is "A Message" and then there is "The Message". Some say as long as you have a message and you are getting in from of the customer good things will happen but I believe you need to have the right message when you get in front of your potential customer.
Inventory is your biggest asset! It's exactly why your potential customers contact you. It's one way of grabbing your customer attention with each and every email.
Have you considered a true email marketing campaign?
It's not so much that the 1 liners WORK it's that you are answering the customer question and getting to the point. What you need to ask is..what happens after these 1 liner emails? Are you getting these customer to the phone or are you answering their questions getting a response and then they disappear?
There are many ways of adding a quick soft value proposition sell inside of your emails. Sounds simple but the P.S. note is often overlooked.