make your ex regret it

keep yourself busy, have lots of fun and go out with your friends!
be optimistic and smile a lot, try new exciting things; a slight change in you will be noticed by everyone around you. people talk and it will eventually get back to your ex even if they dont see you around to notice themselves

always be the better person. dont roll your eyes or ignore them if they talk to you, because that just makes you look bad! only talk to them if necessary, dont make the first move. and keep it should have plenty of other interesting things to do with your friends than spend time talking them. you going off and doing other things and not paying much attention to him/her is bound to stand out in their mind!

keep yourself looking good, change your look or just take better care of yourself, work out, eat healthy etc. a new change in you will get you noticed by everyone, including ex's friends, potential new dates, and your ex

make new friends! keep friendly with everyone especially your ex's friends, if they like you then theyll be happy to see you more often

never fake a boyfriend/girlfriend! people talk and eventually they will find out you made it up which just makes you look desperate and a liar!