Know your characters. Before anything happens at all, know who the main characters will be. The No. 1 character is probably going to be the one who solves the crime. But another key character, remember, will be the victim. Think about false clues.
False clues have to be good clues, though.
Readers want to move around in a mystery. You will need to describe where you are taking them, but since they are anxious to get to the end quickly, you can't waste time puttering around with description and meaningless talk. Make your words work for the story. Crime is deadly serious, of course, but solving mysteries should be fun. Humor brings your characters alive.