Non-actives:
Create a separate campaign for the non-active subscribers—the people who have not unsubscribed but have not opened an email in six months or more.

Free email addresses:
People discard their free email accounts quickly—the most popular are from aol.com, yahoo.com, gmail.com, and hotmail.com—and usually access them with a browser instead of an email client like Microsoft Outlook. Most of your email database will consist of these accounts. Over time, these accounts will turn non-active more quickly than other accounts. Consider keeping separate stats for them as well as fashioning separate campaigns designed to be seen on a browser with a cluttered screen.

Optimize for mobile:
It's likely a sizeable portion of your recipients read your email on their mobile device—and that portion is probably growing. Optimize your email for the small screen with a narrow format and large buttons to counter the "big thumbs" syndrome. That design will also work on the desktop screen.

Auto-responders:
Your email platform probably has an auto-responder you can use, which means you can automatically send a pre-authored email (or series of them) when someone takes certain actions, like signing up for the first time or purchasing a ticket. It's another way to stay in front of your audience. Create a "Did You Know" series about your venue or special biographical tidbits about performers.

Keep it fresh:
Freshen up the creative for your active subscribers fairly frequently to avoid the I've-already-seen-that trap. Revise your newsletter to add a different graphical element every few months and revise your marketing blasts every couple months.