How to think when saving application information (settings, data etc)?

Been thinking about this some time: Let's say I have a application where you can add and use reminders.

What is the best way to store this? In the past I've always used a textfile but it can get problematic if I later want to add another "field" to each reminder in the textfile. Let's say I add an feature for recurring reminders.

What is the most volatile way? Text? Xml? Json? SQLite?