Excerpt from this article:
What makes an effective trigger? How can you be sure that the notifications you’re sending are welcome and lead to higher engagement instead of driving users away? Below are a few tenets of notifications that engage users, instead of alienating them.
Good Triggers Are Well-Timed
…For instance, imagine you have a connecting flight and only 40 minutes to spare. As soon as you land, you’re worried about which gate to go to next and how long it will take you to get there. You turn your phone off airplane mode and, voilà, there’s a notification from your airline with all the right information. Your boarding time, gate number, and whether your departure is on time are presented at the moment you’re most likely to feel anxious. Now you can get to your next connection without having to frantically scan one of the terminal’s crowded departure screens. By providing information at the moment the user is likely to need it, the app builds credibility, trust and loyalty.
Good Triggers Are Actionable
Good triggers prompt action while vague or irrelevant messages annoy users. It’s important that a trigger cue a specific, simple behavior.
…The intended action prompted by the notification can also occur outside the app. Google Now tells users when to leave for an appointment based on what it knows about their location, traffic conditions and mode of transport: “Leave by 11:25 am to arrive on time.”
Good Triggers Spark Intrigue
…Timehop, for instance, sends a cheeky notification reading, “No way, was that really you?,” and prompting the users to open the app. To see the photo, users need to simply swipe. It helps that Timehop’s messaging is lightweight and humorous enough to be out of the ordinary.