Here’s a tip. Your audience is not “everyone who likes a good mystery.” Seriously.
2 - Make sure that every single character in your story has a motive. And I don’t mean a motive for committing the crime. I mean a reason for every single thing they do. Not just a reason, a believable reason. Because in real life, we all have reasons for the things we do -- in fact, we often have several reasons, even if we appear to be behaving inexplicably and unreasonably.
If your characters are acting out merely in order to advance the plot, go back to the drawing board and figure out a plausible reason for their actions. And then lay the groundwork so that your reader believes this character would act that way given this certain set of circumstances.
Here’s a tip. “In real life people act out of character” is not sufficient motivation. Fiction has to make sense in a way real life does not.
3 - Have at least three viable suspects for the crime. Of course, when the book starts out, everybody is a viable suspect, but by the mid-point of the story, most readers have ruled out most of the suspects. Keep it entertaining for the reader and make sure there are at least two -- preferably three -- viable suspects remaining. Which of course goes back to making sure everyone has believable motives for the things they do.
Here’s a tip: Good people do bad things. Don’t be afraid to make your villain sympathetic or even likable. Everybody has a breaking point, and maybe your likable, sympathetic culprit got pushed too far.
4 - FOR THE LOVE OF GOD, PLEASE HAVE SOMEONE DO SOME GENUINE INVESTIGATION. All useful information can not come via a cop boyfriend. Also, deductions must be based on information received either through questioning or observation. Crimes cannot be solved solely by acts of God, coincidence, psychic powers or intuition, though these things may play a role in the denouement. Your sleuth’s flash of intuition must be triggered by something that the reader would also have a fair chance of connecting to the crime.
Furthermore, the villain cannot just give up and confess. As convenient as this would be, no. Really.
Here’s a tip. Think of writing your mystery novel as a friendly game played between you and the reader. You want to make it fun for the reader, so it can’t be too easy. But you also can’t cheat and withhold vital information. Readers do want to solve the mystery -- they want that ah ha! I knew it all the time! moment -- but they don’t want it to come too far ahead of your sleuth.
5 - Don’t focus on the puzzle and the twists at the expense of the emotional core of your story. Murder is a terrible, terrible crime and your story should reflect that. Don’t let your victim be a cipher. Whether the victim was someone everyone wanted dead or beloved Uncle Arnold, there should be ramifications to this death, there should be emotional impact. Murder is a serious business and your story should reflect that -- even if you write funny, cozy mysteries.
Here’s a tip. You should give as much thought to the characterization of your primary murder victim as you would any other cast member.