Murder Mystery Game?

We are having a 4th of July party and after everyone eats and we let off fireworks we usually play some type of game. I am very interested in a murder mystery game, but we've never actually done one before. Anyone with any experience or tips they want to share? I'm looking at buying one of the printable games off ebay--phantom cruise I think is the name of the game. Any info, input, or advice is welcome!