exactly what i do - i always know that it's happening, and i always know that it's only temporary, but when i get this way, i allow myself to acknowledge it, and wallow a bit. usually, i'll go out drinking by myself, sit in a corner ignoring people (except for the occasional glance around the room) and just let my mind go where it will. typically, by the next morning, i'm ok. ocassionally, it'll last 2-3 days, but ever since i overcame my clinical depression several years ago, it's never been more than a few days at a time.
now that i think about it, one of the realizations i had was related to the "why are we here? what's the f**kin point? why bother?" questions... it finally dawned on me (as other people have mentioned) that we're here to have fun, and enjoy ourselves. think about animals, or little kids - think about the silly things they do; dogs will chase their tails, cats will tear through the house chasing nothing (or each other), little kids will spin themselves around & around & around until they're so dizzy they fall down laughing, then get back up & do it again!
in fact, that's exactly what i did - when i realized that, i went in my front yard & spun around until i fell over... it was surprisingly fun, once i got over the idea of it looking silly for a grown man to be doing it! anyway, just be goofy/silly for no reason - everyone takes life too seriously & never takes time to do the stupid, funny stuff.