As I write this essay, the founder of Email Data Source is telling the audience at the Inbound Marketing Summit, that email marketing has a return on investment of 44:1. I believe that, and Bill McCloskey’s words remind me that it’s been a while since I sang the praises of this venerable but highly useful marketing tool.

E-mail should be central to your online marketing plan. It’s how you turn casual passersby into steady customers. It gives you permission on a regular basis to contact your constituents. It’s your best tool for driving website traffic and business results.

As a practitioner of e-mail marketing going back nearly a decade, I’ve learned a few simple do’s and don’ts. Fortunately, there aren’t a lot of rules. The most important ones are to be useful and to respect the access that your subscribers have granted you.

Do give visitors to your websites every chance to subscribe to your e-mails. Put a signup form on every page. If you can manage it, squeeze a promo into your e-mail signature. Remember, a Web contact is casual but an e-mail subscription is a relationship.

Do give your subscribers special treatment. Offer them exclusive offers and discounts. Some software companies now give newsletter publishers free promotional licenses to products that are one release out of date. Look for these offers and ask if you can adapt them for your subscribers.

Do use an e-mail service provider. I use iContact, but there are many others, including Constant Contact, Benchmark Email and Lyris. There are even free options. For a nominal cost, you’ll get reporting, tracking and list management you’d never be able to duplicate yourself.

Don’t deceive your subscribers. If you tell them they’re signing up for a newsletter, don’t send them promotional messages. If you say you won’t contact them more than once a month, then don’t do that. Monitor your unsubscribes. If a lot of people are leaving, they’re trying to tell you something.

Do provide a Web version of your newsletter. Mine is here. This makes it easy for people to share your content on social bookmarking sites, Twitter and Facebook. It also makes you discoverable by search engines. Finally, it’s a way for people to respond to you.

Which reminds me: do invite response to that Web version you just created. Email is boring when it’s one way. Start a discussion.

Do sweat the subject line. Make it provocative or intriguing. However, don’t mislead people into opening the newsletter if you can’t deliver the goods.

Do keep messages brief and varied. Provide several “points of entry” to engage your audience’s different interests. Have fun. The most well-read item in my newsletter is the short “Just for Fun” blurb at the end. Do you think I don’t know that?

Do provide alternative delivery in text format. All service providers support this option. Not all subscribers prefer HTML and they shouldn’t have it forced on them.

Don’t add subscribers without their permission. There’s nothing wrong with renting an opt-in list, but scraping addresses off websites or borrowing other people’s lists can get you in legal trouble.

Don’t underestimate the value of e-mail marketing. This newsletter consumes three to four hours of my time every week. I wouldn’t do it if I didn’t think it was important.