A really good after-party will generally ruin your next day on the polo fields.
If you're a hot chick in LA on lots of promoters' lists, you'll get invited to some parties.
If you're a dude in LA with connections to sponsors and promoters of the parties, you may score some invites.
Most of the time I've been at or around these supposed "good" after-parties, they're full of douchebags that didn't go to the show that day. Stick with your friends, or new friends or the campgrounds if you've still got steam after the show.