When you bump into your ex, you want to give the impression that you're happy but busy, even if that's not strictly true. If your ex sees you moping around in the mall, sad and depressed, they're definitely not going to believe that you've moved on.  When you see your ex, smile a few times at them, but not in a flirty or longing way. You want them to think that you're happy because your life is good, not because you saw them. Try not to let any of the unhappiness you experienced during and after the breakup ruin your mood. If your ex asks you about how you're doing, focus on only the good, not the bad. That competition you won last weekend? Tell them. That school you got accepted into? That too. Don't lie, but focus on your accomplishments rather than failures. Don't make time for them. This will make them think twice. Say something like, "Oh, I'm just about to meet a friend, good talking to you," and walk off without looking back. Do this relatively early on. Nothing says "I'm desperate" than when you immediately answer your ex's texts or phone calls. Take your time. Take a couple days to respond, pretending that it's really not a big deal. The internet is a great way to convince your ex that you've moved on. Facebook, Instagram, and Twitter are all golden opportunities to publicize your newfound freedom.  Take plenty of pictures when you're out with your friends, especially when you're having a blast. Going out to the beach? Take a picture and post a caption with something like "Tamed two beaches in two days with my two best friends. Beach bod, say hello to summer!" Stay a little mysterious. Have your ex and their friends wondering what you're actually up to. Post something a little mysterious to your social network that will get your ex thinking about all the cool things that you're doing without them. Master the humblebrag. Humblebragging is when you pretend to be humble but actually brag. It sounds a little something like this:  "Ugh, my hair is sloppy, I have no makeup on, and I'm wearing sweats and...

TL;DR in Chinese:
心不在焉地对他笑。 不要着急回复对方的短信、微信，或者电话。 在社交网站上晒晒你最近参加的活动或新结交的朋友。 不要让他看出你还在乎他。