I understand that the letdown after a con is rough; I go through it for a few days every year after AX. But to call it a depression is a bit of an over exaggeration don't you think? Depression after all is a very serious medical condition and I'm pretty sure that someone suffering from depression is suffering far more than many of us are for the few days or week after a con.
Sorry to be preachy, but I have a couple friends suffering from clinical depression and it just rubs me the wrong way when people compare the return to the real world after a con to a 'depression'...I'll step off of my soapbox now.
That said, to counteract the letdown I would suggest catching up on your anime, start planning new cosplays (which I do after every con now) or hang out/keep in touch with con friends as much as possible. For me, the key is to pack my schedule full of other activities throughout the year and before you know it, it's time for another con!
Anime Los Angeles
Potential Future Crossplays
Maki Nishikino (Love Live) - Summer school uniform