Gossip Girl here. Your one and only source into the scandalous lives of Manhattan's elite. Here on the Upper East Side, only the most seasoned of hostesses know that your party is only as succesful as both its pre- and post-party. Yes, darlings, you heard right: pre-party. While a fair share of hit parties are known to host post-parties, here in the Big Apple, just one publicity stunt simply won't do. Everyone knows a pre-party is mandatory for a successful party, and that includes our…