Game State Management.cs

/* GameStatus.cs */
using UnityEngine;
using System.Collections;
using UnityEngine.SceneManagement;

public class GameStatus : MonoBehaviour {

	// There are 3 major ways to persist this data between scene changes.
	//  1) Save the info into something persist (PlayerPrefs, a save file)
	//			- This preserves data even between game executions, not just scene changes
	//		** This is a very, very minimalist implentation. Has the benefit of
	//			true persistence. Also, you don't need a single, central class to store
	//			data in this way. Each object could save/load it's own info.
	//
	//  2) Static class data.  Very simple. Occasionally leads to weirdness from
	//		inside the Unity Editor, but not actually breaking things. You could
	//		do really messy "global" style variable with this, but you can also
	//		do really nice encapsulation as well.
	//			** This is the ideal implementation if your persist data
	//				is just that:  Data. It doesn't DO anything. It can
	//				be implemented as a purely static class.
	//
	//  3) DontDestroyOnLoad -- This flags a GameObject such that when we change
	//		from one scene to another, it doesn't get destroyed. (i.e. it is still
	//		present in the newly loaded scene. To use this to the best possible
	//		advantage, you need to use the Unity Singleton Design Pattern.
	//			** This is the ideal implementatino if this class needs to DO
	//			things, not just store things. In other words, maybe you need
	//			it to have an Update function that modifies the game data regularly
	//			and will need to do so on every scene.
	//

	protected int lives = 3;
	protected int score = 0;

	static GameStatus instance;

	public static GameStatus GetInstance() {
        
		return instance;
	}

	// Use this for initialization
	void Start () {
		// "Implementation #1"
		// Load data from PlayerPrefs -- this might be from the
		// previous scene, or maybe even from the previous execution (i.e. saved
		// between quitting and reloading the game)
		//score = PlayerPrefs.GetInt("score", 0);
		//numLives = PlayerPrefs.GetInt("lives", 3);

		// We want to be a Singleton (i.e. there should only ever be
		// one GameStatus instance at any given time.)


		if(instance != null) {
			// Someone ELSE is the singleton already.
			// So let's just destroy ourselves before we cause trouble.
			Destroy(this.gameObject);
			return;
		}

		// If we get here, the we are "the one". Let's act like it.
		instance = this;	// We are a Highlander
		GameObject.DontDestroyOnLoad( this.gameObject );	// Become immortal

	}
	
	// Update is called once per frame
	void Update () {
	
	}

	void OnDestroy() {
		Debug.Log("GameStatus was destroyed.");

		// Before we get destroyed, let's save our data to our save file.
		// This is "Implementation #1".
		// This will happen whenever this object is destroyed, which
		// includes scene changes as well as simply exiting the program.
		//PlayerPrefs.SetInt("score", score);
		//PlayerPrefs.SetInt("lives", numLives);


	}

	public void AddScore(int s) {
		score += s;
		// We could take this opportunity to save the score in a file/UserPrefs
		// But depending on implentation, this could be slow/inneficient.
		// On the other hand, if we are constantly saving the data on 
		// the fly, it means that we pretty never lose anything to a crash.
	}

	public int GetScore() {
		return score;
	}

	public int GetLives() {
		return lives;
	}

}


/* ScorePanelUpdater.cs */
using UnityEngine;
using System.Collections;
using UnityEngine.UI;

public class ScorePanelUpdater : MonoBehaviour {

	void Start() {
		
	}

	void Update () {
/*		// This a (relatively slow/bad) way to grab the GameStatus
		GameObject go = GameObject.Find("GameStatus");
		if(go == null) {
			Debug.LogError("Failed to find an object named 'GameStatus'");
			this.enabled = false;
			return;
		}

		// It's the GameStatus script that we actually care about.
		GameStatus gs = go.GetComponent<GameStatus>();

		GetComponent<Text>().text = "Score: " + gs.GetScore() + " Lives: " + gs.GetLives();
*/

		GetComponent<Text>().text = "Score: " + GameStatus.GetInstance().GetScore() + " Lives: " + GameStatus.GetInstance().GetLives();

	}
}

Youtube source: https://www.youtube.com/watch?v=WchH-JCwVI8 (1hr 10min)