Why do you think people suffer from post-anime depression? It's nothing to be ashamed of, IMO--it just means you become attached to the show and it was probably a good show or at least a show suited to you if that's the case. Just don't let it get ahold of you too long--that can be bad.
I've suffered it after watching Dragonball Z, Code Geass, Sword Art Online, and The Pet Girl of Sakurasou. Without getting into too much detail, what usually hits me is the sense of comraderie--a long-lasting friendship built over the course of the show. It's sad to see the characters go. I want to see them interact with each other and have more fun. In pretty much all instances, that was the case--however, I'd say Code Geass was more than that. While the student council definitely had its bonds, it was naturally also sad to see certain characters die, and when you compare the cast at the beginning of the show to the end, it's changed so much it really does kind of hurt.
One thing I like to think in this case, especially to anyone who think it's silly... is that though characters in a show might be fake, just as actors in reality TV shows are, in their fake world, they're real. And to enjoy the show you need to invest yourself into the world and the characters and imagine them as real. In some shows that's really hard, especially if the show itself doesn't take it seriously enough, but in others it's easy to feel the weight of people's lives... and deaths.
Aside from all that, what do you do to deal with PAD? A lot of people say "watch another anime" but an endless chain of PAD is kind of lame, lol. I like to dwell on it a little but that usually hurts me since it distracts me from real-life, silly as it is. And it definitely gets to me a bit. Doing other stuff helps, as does just keeping yourself busy with totally unrelated things, and I found doing exercise helped a little as well.
