Viral marketing is not an objective: It's an integral park of a campaign strategy that is used to achieve objectives. It is not an add-on. You should craft your message in a manner such that it encourages pass-along. Producing a message with a quality offer or an incentive for pass-along is what viral marketing is all about. Merely suggesting that email recipients forward your message to others is not viral marketing. Adding a line at the bottom of your email that reads "Feel free to forward this message to a friend" is not viral marketing at its best.
Offer something worthy of sharing - a valuable discount, vital information - or offer an incentive for sharing - additional entries into a sweepstakes or an added discount or premium service - and viral marketing happens naturally... and often quite successfully.