Remind consumers of the benefits and special features that come along with subscribing to your lists or newsletters, such as access to preferred sales and exclusive promotions. Give them something they'll miss if they decide to opt-out, and highlight these benefits in a list with bullet points.

Macy's welcome email highlighting the benefits of subscribing to their list

Setting subscriber expectation
I have fallen victim to signing up for a newsletter advertising content I am interested in and then being bombarded with multiple emails each day, hindering my ability to fully read any of them. Give consumers a choice of how frequently they want to be contacted -- daily, weekly or monthly. If this is not an option, at least set their expectations up front so they know how often they will receive publications -- and then stick to it!