Building a white hat email list

I've seen commercials for constant contact and thought, wow, they are just making you pay for something that is free...

I have a service site that requires people to register their email with us in order for their orders to be processed.

What is the most effective method of building this into a list that I can send out monthly promotional offers to?

I pay good money to get these leads from adwords, return customers is key.

Thanks