This is something that is meant to prick the interest of your readers enough to increase click rates. So pick a subject line that matters to your readers and is relevant. Use a catchy front line, ask a question you think your users might want to see, the latest update on a top pick product, or create urgency by putting a deadline on a particular offer. A few best practices are using the language your audience is familiar with, keeping it short for mobile optimization, use clear readable words, and personalize your messages.