I was pretty damn sad two days ago, on the last day of school. I had to say goodbye to a few friends, some possibly forever. I was fine until my friend Nazifa, a close friend who I really really really love. And when I saw her, I realized that this would most likely be the last time we ever managed to see each other, and that really got to me. We hugged and after me giving a few words to her ear "Always be amazing and never ever give up" I kissed her and watched her walk away. And then I started crying.
That's basically my answer. If it's something as emotional as that, I cry. I don't have shame in saying that at all. Sure, I started crying in the corner and my friend sorta had to put her arm around me and cheer me up. But it does help. After that, I simply recall all the good times and hope for everything it's worth that I see her in the future.
For less monumental moments, I just listen to music or lay in bed staring at the ceiling.