Listen.
Let them get it out of their system and don't rudely interrupt them with "wisdom" before they even finish letting out all the pain they feel and thoughts running through their mind.

Pick up a hobby.
Take your mind off the ex. Learn a new skill. Just to show the ex you're capable of doing things without them and how awesome and talented you are. Time heals all, and with all that time...might as well make yourself better than you already are.

Don't dwell and learn from the experience.
Can't really elaborate on that. Not easy to do, but not impossible either. I'll always nudge them in the direction they should be heading towards.