To get press to cover your event, you need to have something press-worthy, so simply start there. If it's a product launch, that may not be enough. You need to have something that is going to make it press-worthy. Be it a really cool DJ (but a celebrity DJ), or be it a celebrity host, or be it a really cool performance. There's got to be something press-worthy to even get the press engaged to even want to cover it.
Once you get that press-worthy element, it's time to notify the press. The best way to notify the press is by sending out a media alert. Basically, the media alert is a who, what, when, where, why, and why they need to cover it. There's two ways of getting press to cover your event. Press to cover your red carpet, and they stay out on the red carpet. Then there's the press that you want to come inside, observe the party, and write about the party, the event.
Once you've gotten that press-worthy element, which is like your headliner or your host or your cool DJ that goes along with whatever product you're launching or event you're doing or magazine or whatever it happens to be, then you also want to supplement the crowd with other celebrities. You want to get other celebrity guests in the room. You can use this press-worthy event that you've already crafted and send out an invite to get other notables in the room.
Then, when you want to reach out to your red carpet to fill that with media and press, you send them a media alert that says who's going to be there, who's going to be walking the carpet, who they're going to get to talk to, who they're going to get to photograph.
Another way to get press for an event is to know the writers that write about events, the party pages, the gossip reporters. Those are the people that you let inside and let them observe and write about the event.