Ask users to authenticate only in exchange for value, such as personalizing the experience, accessing additional features, purchasing content, or synchronizing data. If your app requires authentication, keep the sign-in process quick, easy, and unobtrusive, so it doesn’t detract from the enjoyment of your app.
Delay sign-in as long as possible. People often abandon apps when they are forced to sign in before doing anything useful. Give them a chance to fall in love with your app before making a commitment to it. In a shopping app, let people browse your merchandise immediately upon launch and require sign-in only when they're ready to make a purchase. In a media streaming app, let people explore your content and see what you have to offer before signing in to play something.
Explain the benefits of authentication and how to sign up for your service. If your app requires authentication, display a brief, friendly explanation on the login screen that describes the reasons for the requirement and its benefits. Also, remember that not everyone using your app has an account from the start. Make sure you explain how to get one, or provide a simple in-app way to sign up.
Minimize data entry by showing appropriate keyboards. When asking for an email address, for example, show the email keyboard screen, which includes helpful data entry shortcuts.