Lavinia.
On average, what is your return per £ 100 spend?
The reason I ask is that I was surprised recently when I checked some figures for postal "reminders". I found I had an 8:1 return on investment (ROI) excluding cost of performing the cleaning.
It tells me I should be churning letters out far more frequently than I am.
With emails, you may have to make it more than just offers to hold their attention. Why not get a customer who runs a business to supply you with some vouchers (a bit like your own version of Groupon)?