Treat Them The Way They Treat You
You don't withhold affection when they're being affectionate with you. You will reciprocate it. Savor the sweetness of being in each other's company. You will not act ugly in any way, shape or form.
When they withdraw, you will do the same. Give them the space so they will miss you again.
When they're doing something that doesn't sit too well with you, you will let them know by ignoring them. When they are showing remorse, you will reward them with your warmth, just enough that they will know that they will need to show an effort to win your affection.
Familiarity breeds contempt. Always wear an aura of mystery. Be a little unpredictable in your response but not too much you become erratic and more like a loose cannon. You want to appear as someone who is in charge of his/her happiness, yet soft enough to be vulnerable every now and then.
You attract and intrigue with the strength of your courage and self-confidence, and you emotionally bond with them with the tenderness of your vulnerability: when you open up and express your deepest feelings, longing and fear -everything that makes you human.
Those are lethal combination that will make your ex latch to you and never let go, again.
To wrap it up, here's how you seduce your ex back.