Convey urgency, pique their curiosity, use an actionable verb and personalize it with the reader's name. 67% of emails are read on mobile phones, so keep it short -- ideally less than 40 characters.
PRO TIP: Be specific about the content in the email, using numbers where possible. For example, "Join 750 others at [Name of Event]". Make your people feel special and valued with subject lines like: "My gift to you", "You're invited!", "Private invite."