If the thoughts are constantly coming up about your Ex in your mind and you have started missing him and you want to make your Ex want you back, then the best way is to apply the law of attraction in three ways:

Rule # 1 Curiosity will develop Curiosity

Whenever you write your emails and SMS, put the curiosity factor in it and see how he will get that attraction. It is a human tendency that if there's any kind of secret thing, you will try your best to know it. You can use this trick on the phone as well. You can tell him something like "I am going out tonight with a good friend of mine, bye... in a hurry" Cut off the phone immediately and don't reply. Once you do not describe the person you're going out with, he will get more desperate to know and your Ex will try to hunt for that person. Once you don't answer his calls, he will feel the urge of knowing it all the more. His inquisitiveness will make you feel better and the same will cause attraction. I bet this surely works.

Rule # 2: As you were social before when you were in relationship, similarly start being social again

Stop being indoors and party or socialize. Look happy and feel happy. In this way he will see you and feel insecure that you can do without him and this make your Ex Want you back.

Rule # 3: Passion will create Passion

Start doing the passionate things in life that you always missed before. Enjoy with no sad thoughts and tell him about the activities that you are doing every day. Also talk about the good time that you spent together. Your Ex will find out that something's amiss and this will make your Ex Want you back.