Last week, I defined viral marketing and suggested why you should consider using it. And in next week's final installment, I offer a list of 13 best practices for successful viral email marketing.

When used responsibly, viral email marketing can be a powerful community-building, brand-building, and list-building tool.

But gone wrong, it can result in legitimate campaigns being perceived as Spam, reflecting poorly on your company and your brand. But there are things you can do to keep yourself out of trouble and ensure better results from your viral email marketing campaigns.

#1 — Make sure it's appropriate to add a forward message. The first step is to consider whether you should even mention forwarding at all! Some campaigns are designed for communicating private information to a specific group of people and are just not suitable for sharing.

#2 — Make the ‘call-to-forward' appropriate to your campaign objective. If forwarding will help you achieve your primary goal, make your forward message strong and visible within your layout. If forwarding is less important, consider making your call-to-forward subtle. Don't distract readers from your main message unnecessarily.

#3 — Remind readers to forward appropriately. Recipients who forward may not stop to consider that someone else might not want to receive the email. If they send it anyway, your email campaign will be perceived as unsolicited, the receiver will be annoyed, and it will reflect poorly on you. Be sure to remind readers to only send your email to people they know will want to receive it.

#4 — Be wary when offering rewards for forwarding. While offering incentives for forwarding can be highly rewarding—for both you and the forwarder—it may inadvertently encourage inappropriate forwarding. If this happens, the backlash could affect your current subscribers as well as potential new subscribers. As with any marketing campaign, it's a good idea to test your incentive on a small group of people first.

#5 — Think about what you want new recipients to do. Think back to your campaign objective. Forwarding alone helps increase awareness. But if you want new subscribers or more sales, make sure there's a call-to-action for new readers (e.g., include a link to your sign-up page or online store).

#6 — Think about how you want new recipients to feel. Read your own email as though you had received it from a friend. Is there enough context provided? Does it make sense? Review your message to address the needs of any new recipients, not just the people on your original list with whom you already have a relationship.

#7 — Remember your brand. Always include your company name, logo and website address in each and every email that you send. You might know who you are, and your regular readers will recognize an email from you, but don't assume that new recipients will have the same knowledge (see #6).

#8 — Choose the right email format. Although rich media is the format we most often associate with viral marketing, it's not the only way to accomplish your objectives. Your email campaign will be most successful if you choose an email format preferred by your audience—after all, you first have to get your email opened and read. After that, it's the content—and a reader's ability to relate to it—that will get your email forwarded.

#9 — Use email marketing software that measures viral activity. When you use some email marketing software, you can embed a ‘forward' link in your campaign that measures: how many people forwarded your email, how many times each person forwarded it, the number of additional click-throughs gained by forwarding, and which people became new subscribers as a result. Measuring success is a critical part of email marketing, and forwarding success is no exception.

#10 — Make it clear that readers should use the forward link in your email message, not the forward button in their email client. Measuring viral activity is only possible if your readers click on the forward link you embed in your email message. If they use the forward button on their email client, all tracking ability is lost. Since email was around way before email marketing software, you'll need to make your forward link obvious and your call-to-forward clear.

#11 — Craft your call-to-forward carefully. Your forward link is like any other trackable link—it appears as a clickable word or phrase. If you want people to click on your forward link instead of their forward button, say more than just “Forward to a Friend.” Spell it out for readers with words like “Click here to send this email to others you think would enjoy it,” or “Click here to share this valuable information with colleagues.”

#12 — Vary the placement, size, or color of your forward link. If you send a regularly scheduled mailing, text that appears in the same place in each email can become “invisible” to the regular reader. Instead of always putting your forward link in mouse type at the bottom, move it to the top, or use a bigger font or different color for a while to get your link noticed.

#13 — Learn from someone else's mistake. Do your research. Read success stories. Learn from the mistakes and successes of other email marketers. This article is a great start, and here are a few of our favorite viral marketing case studies, from MarketingSherpa.com:

Singapore Airlines Tests Viral eCard Campaign. Can Viral Games Work for Business-to-Business Marketing? Weekly World News Raises Newsstand Sales 15% with Web Site Publicity Stunt.