As often as possible is the best answer. Realistically, you’ll want to perform list hygiene before each send (or continually if you send daily).
There is a 30% loss on subscribers throughout the life of an email campaign on average. If you don’t take care of your lists, you risk losing the other 70% due to a hit to your sender reputation.
Advice on breaking through to subscribers?
You have to answer the question, “What’s in it for the subscriber?” — If you can’t answer this beyond “We want to get sales,” you’re in trouble.
Set proper expectations up front, and don’t deviate from those expectations without your customers’ knowledge and consent.
Tips for building engagement?
Make your call-to-action easy to find and easy to act upon.
Be responsive to changes in behavior — Make use of a preference center and give your subscribers control.
Don’t waste their time, ever.
Note: The EEC podcast series also includes podcasts on a) Database Growth and Development; b) Deliverability; c) List Growth Offline and Online Tactics; and d) List Growth. Check out the entire podcast series here.