Most Helpful Guy
Here's how: you accept the fact that he made a decision, and it was entirely his to make, and if it really was a bad decision then he will eventually realize it and he will be in more pain than you will ever imagine, but if it was a good decision then he will be a better person for it.
(if you haven't caught on by now, I'll be more explicit: no action on your part can make him feel regret. Regret, in this context, means feeling sad or repentant over one's own past action. ONE'S OWN PAST ACTION. Not your action. His action. Your actions do not qualify as actions that he can regret.)1
Most Helpful Girl
The best 'revenge' is showing them that you're perfectly fine without them.
Resorting to devious tactics in an effort to get them to come 'crawling' back (even though he broke up with you?) is most likely only going to end with him being glad it's over.1