Remember, language is only half of what ESL really is. Maybe less.
Specialize -- don't try to be everybody's ESL expert. Instead, focus on the demographic you as a teacher are most interested in (challenged by?). Perhaps it's K-12 children of immigrants from Southeast Asia. Or, senior PRC-born executives who, after decades of living in America, still feel self conscious about doing and saying the right thing in the American business environment. Then you'll have to undergo the enjoyment (if you're a pro) of learning all you can about your target group's challenges and needs -- and customs, of course. In exchange, you'll be REALLY qualified to assist your students in one adapting from one society to another -- surely one of life's greatest undertakings.
Use modern communication technology -- Facebook, Twitter, other social apps, and portable communication devices. Incorporate these into the heart of your ESL process, as both a means of learning and delivering that knowledge. Today's young people will not put up with 1960's-era cartoon books as learning media.