I am with TickleTrain tool for some month now. it allows an easy followup method for invoices or anything else really. But I do use it for invoices. So anytime I send an invoice to one of my customers, I just include a special email address in the bcc field before I send it- this tells tickletrain to track and follow up on this email for me. It's really easy to use. And it's free! But I think I will upgrade to the paid account now that I have been using it for a while. The free account is limited to 5 followups at any one time. Just visit their website if you'll need more details or interested in their services!