Having a lot of partying people at a place, combined with sensible policemen, is the best recipe for not getting busted. Had that at least twice in my life. Once at a house party with a lot of young, just around the driving age, people. Police showed up, and decided to not bust it, because sending people on the road would be more dangerous then letting things just go on. A few years later, I was at an illegal outdoor tekno party, which also got a visit at 8AM by two policemen. They basically just went up to the DJ and sayed: "We will return at 3PM, and you will be gone."

Yep, but it depends. We once organized a party in a soon to be demolished factory building. And exactly that happened, the police decided to let us be because it would've been trouble to have a few hundred ravers in the middle of the city.

The second time we were not that lucky, it was a warehouse, and they came with flashlights and kicked us out.

Once you really get organized, you pay the police off by hiring off-duty cops to be your security and paying them overly well.