Send e-mail newsletters to current, past, or potential customers on a regular basis.
Make sure the recipients can opt-out if they wish. Sure way to get into spamming list is to send emails nobody wants. Give them news about your projects, games, hints, tips, etc. Make sure it’s on regular basis.
Tip: indiegamebusiness.com (not public at the time of writing) has a system where you can create and send your newsletters and press releases etc. Another one (newsletters only) is yourmailinglistprovider.com