Stefan Pollard offers a few ideas on how you can let your customers voice their frequency preferences:

• Meaningful choice at sign up. From the time that the users inputs their e-mail addresses, give them the opportunity to let you know what they’re interested in. Allow recipients to tell you from the start if they’re interested in receiving new product updates, or if coupons and special promotions are really what they’re after. List each mail’s average frequency so that recipients know what they’re getting into.
• Create preference centers. Most professional e-mail programs allow users to login to their preference centers at any time. Preference centers simply allow recipients to change their minds about the frequency and/or types of communication they receive from your company. Remind subscribers in each message that they can use the preference center at any time.
• Ask during the unsubscribe process. The unsubscribe process is literally your last chance to listen to your customers. Instead of assuming that recipients wish to receive no further e-mail communication from your business, instead offer them the choice to opt-out of receiving particular types of e-mails. Just as on the sign-up page, you’ll want to indicate how often specific types of e-mails are sent, so that the recipient’s expectation matches the reality of the distribution schedule.

Source: CRMbuyer