To give you an idea where the science is going, look to a leader like Procter & Gamble, which has used viral email to launch hit products like Tide Coldwater, driving as much as 50% of all traffic to its site. The company has now gone as far as to assemble a network of 250,000 carefully screened American teens who volunteer to be the first among their peers to gaze at new products and ideas. Code named "Tremor", the unspoken expectation is that the teens will share the news with their circle of friends. P&G's next target is Moms.

If you are getting the impression that building a viral campaign isn't something to do on the spur of the moment, hold that thought. It takes plenty of planning as well as deep awareness of your company's core attributes (brand). A recent book, "Creating Customer Evangelists," by Ben McConnell and Jackie Huba has some thoughtful tips:

• Create community. Don't think of customers as transactions but as a community of like-minded people. Then, address the community, not the transaction.
• "Napsterize" your knowledge. Make it a point to share knowledge freely even if you think it's secret or proprietary because the value of an idea increases in value as it reaches more people. They cite MIT, which is making its entire courseware available for free, such as teachers' notes, presentations and syllabuses. The rationale is that it increases value of the institution by attracting more people. Blogs are one of the most common ways to do this today.
• Build the buzz (like P&G) by disclosing information and sneak previews to invitation-only lists. Maybe they are your best customers. People are more apt to pass on news that they know isn't common knowledge.
• Make bite-sized chunks. If your product or service is complex, devise specialized, smaller offerings to get customers to bite. For example, consultants can create workbooks or deliver seminars to provide a piece of what they have to offer.

These are necessary first steps in packaging yourself and making your expertise more interesting to people who can influence. Next, you can address the actual mechanics of using email as a viral channel. The trick is to start early so that the campaign has a chance of firing on all cylinders - the timing, the copy, the email, the Web site etc.

Step #1. Unique concept. As cool as iPods are, you probably won't stand out offering them as rewards. The easiest way to be unique is to build the concept as much as possible on what you actually do. For example, if you're in the flower business, an attractive offer might be the ability to download your choice of screensavers featuring your favorite flower. Think of what it is you have to offer, how you might individualize it and how to provide a sample.

Step #2. Clear incentive. What's in it for them if they forward your email? An additional entry in a contest for every forward is often an effective motivator.

Step #3. Prominent Forward To A Friend - In the design of your email, make sure the FTF link is right out front and clearly associated with any resulting reward.

Step #4. Web-ready. Email and Web landing pages must work hand-in-glove, to the extent that clicking from one to the other is practically seamless. This means same look and feel. Treat it like you're simply continuing the conversation (which you are).

Step #5. Sending rules. Creating "buzz" works well in an environment of exclusivity. It may be that you don't want to blast your entire list at once, but deploy selectively.

Remember, word of mouth email is one more tool in creating online communities. Properly and creatively used, it can be the best way of finding your best customers.