Some friends of mine threw one about two years ago. It was a blast, though most of us were a little apprehensive about REALLY getting into character. Encouraging people to really play up their roles is probably the hardest part.
As I didn't organize, and only took part in one, my best advice would be to limit your party size. Ours was huge, and so what ended up happening is the planners made a role for someone to basically be the gossip, and spread around what others were saying. When that person bailed last minute, there was no one to spread around what was going on, and so nobody was ever able to solve the murder (My friend, who played my bodyguard, brought a cap gun, and when we all got frustrated over the lack of evidence, he randomly shot someone. Turned out he shot the murderer, so we called that canon).
If your party size is unavoidable, I'd say either make sure everyone get's a good chunk of evidence, or at least make sure you have more than one gossip to spread the word, in case one can't make it.
Literally, in the party I was at, I never once talked to the murderer, or heard anything about the murderer. I only found out later what the story of his character was.
I made up some RCMP "Under investigation" kinda breakdowns of the characters afterwards, got all the notes from the party throwers and everything. I'll see if I've still got them, just in case anyone's curious.
edit:
Foolishly, I apparently neglected to make sure I had them backed up before I removed them from Facebook. So they are gone forever, I suppose.
By my recollection, there were 16 suspects, not counting the victim and the two people throwing the party. But I'm also almost positive I'm forgetting at least one person.
