Come up with a memorable name. If nothing else, try to come up with a unique name for your character so he or she doesn't get lost in the mass of Detective Steeles and Agent Smiths and Investigator Blacks out there. Names like Stephanie Plum (by Janet Evanovich) and Hieronymous "Harry" Bosch (Michael Connelly) immediately catch the reader's attention because they're unique and unusual. If you're writing in a crowded genre like mystery and you're writing a protagonist with a common, expected job, try to make their name uncommon or unexpected so they don't get lost in all the other noise out there. Incidentally, this is also a great opportunity to exhibit some racial diversity in your writing; Anglo-Saxon and names of other Western-European ancestry are fairly common in American society... but creating a character of Asian, African, or Middle Eastern descent, for example, could add depth to your character and also provide them with a wealth of names that will stand out against all the Jacks, Johns, Bills, and Mikes in contemporary American fiction.

Come up with a memorable twist on the job. Rather than a unique name (or perhaps in addition to), try and come up with a unique take on the actual job itself. Faye Dunaway (and Rene Russo in the remake) is an insurance investigator in The Thomas Crowne Affair. John D. MacDonald's Travis McGee character is a "salvage consultant." Going back to Janet Evanovich's "numbers" series, Stephanie Plum is an amateur bounty hunter. All of these roles are interesting twists on the investigator archetype, and in many cases, the specificity of the job provides ample opportunity for fresh stories that aren't all the same murder, kidnapping, robbery crimes that so many detective characters get stuck with.

Come up with a memorable twist on the character. Rather than a unique name or twist on the job (or, again, perhaps in addition to), try coming up with a character quirk or personality type that is out of the ordinary. So many detectives are tough, cynical, perceptive, etc. that the personalities of different detectives can run together if you read a lot of mystery fiction. Try coming up with something unusual, like the fact that Adrian Monk (from the television series Monk) is brilliant, but also has debilitating Obsessive-Compulsive Disorder and a laundry list of phobias. Veronica Mars (in the television series of the same name) is a high school student who helps her private investigator father solve crimes (while still attending school and helping with her own classmates' problems). And Liv Moore (from the television series iZombie, by the same guy that created Veronica Mars) is a sentient zombie who works in the coroner's office so she has access to brains without having to kill anyone... and solves crimes by getting flashes of memories (and taking on characteristics of) the victims whose brains that she eats.