Email marketing can play a crucial role in conversions, and an important factor of many online marketing campaigns. Often overlooked is that you shouldn't just consider email marketing as a once/month push - I've considered setting up an auto-schedule of emails for new sign-ups to my sites. As soon as a new user enters onto your email list they should enter a cycle of emails which get sent out--above and beyond the initial email. I'm not talking about spamming but some helpful emails can really help users convert.
Of course there are several other ways to increase sales conversions but this is a basic one for all the new comers out there looking to break into the email marketing world. If you have any other quick suggestions, feel free to chime in.
All the best,