If there's policework involved, make sure you know the real procedures. Whatever form of celebratory debauchery fits your style. Craft a strong hook All novels need effective hooks: the reader should be interested to uncover more from the first page or even betterthe first line.
Create an unusual detective "It doesn't always have to be a police detective in a murder mystery - it can be a psychologist, journalist, lawyer or teacher, say, instead. Tension should he held within the novel and there should be no longuers of boring interrogation.
Some authors pants their way through a mystery, fueled by the tortured screams of their imaginary friends. The answer to the mystery needs to feel like realization, not revelation.
This makes it more shocking.
Suspense in a mystery novel is key. Put in the hours at the library.
Sometimes they overlap. The first page sells that book.