You can use NSUserDefaults to save information and retrieve it next time when the app launches.
https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSUserDefaults_Class/index.html
For example:
NSUserDefaults.standardUserDefaults().setObject("mynameisben", forKey: "username")
let userName = NSUserDefaults.standardUserDefaults().stringForKey("username")
Update for Swift 3+
UserDefaults.standard.set("mynameisben", forKey: "username")
let userName = UserDefaults.standard.string(forKey: "username")