Options that will not add to the cost of a periodical may include: 

If using email for renewals, don’t use it only for renewals, consider sending extra/color photos (if Copyright release is available), timely tidbits, in-depth coverage, save the dates, quote of the week/month, etc. – things to encourage opening over discarding.

This section is drawn from http://www.gregathcompany.com/marketingsubscriptions.html