Good email campaigns lead prospects quickly to the desired end result (=buying) while presenting additional reasons to buy, information to overcome objections and post-purchase reinforcement for those who have already purchased.

A good email campaign:

1.  Inspires a sense of urgency — it isn't enough that prospects buy, they need to buy NOW, and a good campaign presents valid reasons and incentives to motivate prospects to action immediately or as soon as possible.  Deadlines/expiration dates, limited-time offers and free bonus rewards are good examples of urgency-inspiring tactics, but it's important to observe deadlines you advertise to avoid undermining the urgency you are looking to inspire.

2.  Presents a series of justifications for making the purchase, each of which is individually valid and compelling, presented in an order that builds upon previous emails whenever possible.  Each email offers another reason to buy or highlights another aspect of what you're selling, increasing the prospect's motivation to buy.  Also, the emails, when read as a series, should "tell a story" (i.e., follow each other in an order that makes sense, with each expanding on and/or summarizing previous emails).  Each email's selling point should be sufficient to stand alone, yet when woven together, the individual emails should present a compelling case for purchase, more compelling than any of the emails alone.

3.  Gives information to overcome prospect objections, so that while the campaign gives additional reasons to buy, it also disarms objections to purchase, thus pushing prospects toward purchase from both sides (accentuating the positive and eliminating the negative).  This two-sided push, when done right, is a powerful and effective tool.

4.  Reinforces the decision to buy for those who have already purchased, and presents that reinforcement as another potential reason for prospects still sitting on the fence to go ahead and take the plunge.  Testimonials and members-only benefits are particularly effective in this regard.  You certainly don't want the buyers you already have to change their minds, and you want those who are considering purchase to want the additional benefits you offer to buyers, so this is another important component to a successful campaign.

To include each of these in your email campaign requires only some advance thought and organization, and they will exponentially increase your results.