How to make data I enter in a UITextField inside a UIAlertController persist through resets in Swift?

I have been trying to integrate a Pinboard bookmarks view (by parsing an RSS Feed and displaying it in a TableView) in my browser app. To get the username and API Token for the feed I have a UIAlertController in the Settings view of my app. The details entered are preserved through the session but if I force quit the app from the multitasking view, The details are deleted. How can I make them stay?

This is the code I'm using for the UIAlertController:

@IBAction func pinboardUserDetailsRequestAlert(sender: AnyObject) {
    //Create the AlertController
    var pinboardUsernameField :UITextField?
    var pinboardAPITokenField :UITextField?
    let pinboardUserDetailsSheetController: UIAlertController = UIAlertController(title: "Pinboard Details", message: "Please enter your Pinboard Username and API Token to access your bookmarks", preferredStyle: .Alert)
    //Add a text field
    pinboardUserDetailsSheetController.addTextFieldWithConfigurationHandler({(usernameField: UITextField!) in
        usernameField.placeholder = "Username"
        var parent = self.presentingViewController as! ViewController
        usernameField.text = parent.pinboardUsername
        pinboardUsernameField = usernameField
    })
    pinboardUserDetailsSheetController.addTextFieldWithConfigurationHandler({(apiTokenField: UITextField!) in
        apiTokenField.placeholder = "API Token"
        var parent = self.presentingViewController as! ViewController
        apiTokenField.text = parent.pinboardAPIToken
        pinboardAPITokenField = apiTokenField
    })
    pinboardUserDetailsSheetController.addAction(UIAlertAction(title: "Cancel", style: .Cancel, handler: nil))
    pinboardUserDetailsSheetController.addAction(UIAlertAction(title: "Done", style: .Default, handler: { (action) -> Void in
        // Now do whatever you want with inputTextField (remember to unwrap the optional)
        var parent = self.presentingViewController as! ViewController
        parent.pinboardAPIToken = pinboardAPITokenField?.text
        parent.pinboardUsername = pinboardUsernameField?.text
    }))
    //Present the AlertController
    self.presentViewController(pinboardUserDetailsSheetController, animated: true, completion: nil)

}