Your sleuth should be likable, interesting and resourceful, with a definite personality that includes quirks and personal issues that have yet to be resolved. Your sleuth needs to have a personal stake in solving the mystery.
2. Consider your setting a major character. Use your setting well--its geography and flavor, its contrasting neighborhoods, businesses, parks and restaurants. Set your scenes in various locales to avoid monotony.
3. Occasionally change your setting. If most of the books in your series take place in a small town, you might have you sleuth solve a murder in Manhattan.
4. Your sleuth needs a best friend or confidant with whom to brainstorm. Consider his/her having a nemesis, as well, to up the tension and add red herrings to the mix.
5. A love interest or interests spices up your plot and adds another dimension. While your reader enjoys the puzzle-mystery aspect of your novel, his/her ties to your sleuth are even stronger.
6. Choose your victim carefully. Why was he/she murdered? What connects the victim to the suspects? Why was the second victim murdered?
7. As for suspects, have many, with various motives, and with varying connections to the victim(s). Don’t telescope the identity of the murderer, but let your murderer appear often enough so that your reader doesn’t feel cheated when all is revealed.
8. Secrets relating to the past are like chunks of dark Belgian chocolate in a chocolate brownie. Every character should have a secret or two. Reveal each secret only when necessary. Use them to your advantage.
9. Every mystery should have a theme. Be it a dispute regarding an inheritance, collecting butterflies or coins, each mystery should include a theme that reflects the concerns of the village or the outside world.
10. Decide what role official crime solvers play in your mystery. Even if you’re writing a cozy series, the police must appear in your books. Is your sleuth friendly with the homicide detective? Do they have an adversarial relationship?
11. Sub plots are essential to any novel, including your mystery. They arise from the theme such as a dispute over land development, or from an issue in your sleuth’s personal life.
12. Make sure your personal viewpoint comes through in your writing. You are unique. Your take on the human condition will help make your series stand out.