From what I have noticed since the last sets of changes there are quite a few different ways to get Merlin's Tokens.
1. Helping a Friend Build or Research - when a friend posts help in facebook then you have a single chance to get a merlin's Token. You can help as many ppl as you want but you only get one token. If you help someone build then you wont get a token for helping someone research and vise versa. This token can be saved.
2. Look What I Won - If your friends share that they won something in Merlin's Boxes you have a small chance of getting a Merlin's token by "visiting the wizard". There is only 1 Token available for 1 person so you have to be fast. This token can be saved.
3. Chest Post - sometime in the game you will have a pop up that allows you to post a chest on facebook. Some say it is right after taking a wild, but i think its completely random. Inside this chest is 1 Merlin's Token and only 1 person can get it, so you have to be fast. This token can be saved.
4. Daily Token - Every 24 hours you will get a Chance at Merlin's Boxes but you only get half of the boxes to choose from. If you send a gift you can open all the choices, but some have reported a glitch that doesnt let you return to the mini game (not sure if this has been fixed). If you dont send a gift then you should still look at the names on the other boxes, because you can still get whats there you just cant chose the locked boxes. This token cannot be saved it must be used immediately. NOTE: If you know you have saved tokens and you see that one is missing after using the daily token then just refresh and it will return.
So there are 5 ways in the game to get upto 4 tokens a day. 3 of which you can save for later. Hope that helps you all to understand how to get free tokens better.