Know the firm professionally. Got my foot in the door almost by accident (wasn't looking actively), through an acquaintance. Interviewed for an investment-related position. Had two phone interviews (a pretty thorough resume walk-through and a case), 2 personality tests (MB type II and Team Dynamics) and an in-person. The in-person included one of their almost-famous group discussions on a topic completely unrelated to the nature of the position, another case, and another resume walk-through, which focused on the stuff in the bottom third of the resume. Did a so-so job - it was my first interview in years - and didn't get the offer. Can't say I was surprised with the outcome, unpleasant as it was. Overall, this felt much more like a management consulting interview than a hedge fund interview.
A word on the culture and the quirks that come with it: the interview experience was not unpleasant and at no point did I feel like I was being condescended to. Yes, they ask tough questions. Yes, they do tape the interviews (with your consent). Yes, there is the book of 150-odd Ray Dalio's principles, some of which feel a little rambling and some of which sound a little extreme, but adherence to which was nonetheless evident in the way the interview was conducted. But the folks I spoke to were smart, they did listen carefully, and they made sure that I got my chance to speak.
So come in with an open mind, no expectation for softball questions, and be prepared to talk for half an hour about that stupid summer job you did after your freshman year.