And yes, always allow the customer to unsubscribe from future mailings.

If you are selling products that have predictable lifecycle (eg,. coffee, chocolates, shampoo), and you can roughly predict how long it will take before the customer will want to buy the product again, then you should follow up via email around that time. This too can be achieved via automated tools like the one mentioned in the post. This idea is closely related to the second point “Offer a coupon for a second purchase”, but does not have to be limited to first time buyers.

However, I strongly recommend that you provide an “unsubscribe” link in every follow-up email that you send (emails unrelated to the order that has been placed). Else you risk annoying the customer thus resulting in lower customer satisfaction.