It’s human nature to dislike being indebted to others, which is the basis of the Reciprocity Principle. To properly leverage this, ensure that you’re offering your customers/followers/subscribers something of real value.

Many retailers offer a discount on purchases in exchange for an email subscription.
An example of this is Figure 1, from Gap in the U.K., which offers 15% discount as an incentive to sign up for their newsletter.

Savvy consumers know how much their data is worth to brands. Still, they are often willing to exchange it, provided they feel the exchange is equal in value. In the example above, a 15% discount could be highly valuable to someone already perusing their website. In fact, as you can see in Figure 2, Gap has a fairly lengthy subscription sign-up form, gathering extensive information about their mailing list subscribers that will later help them target their email campaigns and send relevant emails.

It is worth noting that Gap only requires new subscribers to submit their email addresses. Evidently, their marketing team decided to rely on the reciprocity principal to gather the rest of the information. When in the position of needing to gain many fields of data from your subscribers, instead of making all the fields mandatory, try to leverage the Reciprocity Principle. By doing this you not only gain information that has been voluntarily given to you, but you also don’t annoy your customers.