Recently I read a crime flash story that reminded me that not everyone knows how to write flash and write it well. There are a few rules of thumb that work well for crime flash and help make a story more than a setup for a punchline.
1. You shouldn't use more than two or three characters in a piece that's no more than seven hundred to a thousand words. Yes, there are exceptions that work but it's a fine line to hold.
2. Simple names for your characters work best. When you have multiple characters, please don't give them multiple names like Amy Jo and Billy Bob. When you have six characters and a dog, each with multiple names, your readers are going to get frustrated trying to keep everyone straight in their head.
3. Stick with one law enforcement agency. If your local sheriff can handle the job, let him. Don't bring in a pair of FBI agents, the State Troopers, and a PI to help him. Good rule of thumb - One good guy, one bad guy. Think High Noon not the OK Corral. And yes, I just mixed Westerns with crime fiction, maybe Die Hard and The Sugarland Express would be better examples.
4. You've got a great punchline for your story. Forget it. Make the story about the punchline not a exercise to get to that great punchline. If you're writing humor, the punchline is a great device, but most crime stories are grounded in the crime or its aftermath and the joke doesn't always work. Readers need a satisfying payoff, not an "Oh, God, he didn't just say that, did he?" ending.
5. Flash is short and fast. Keep it as simple as you can. Pick and choose your words carefully. Use active verbs to keep the story moving forward. People read flash for a fast, smack you in the face story. They don't want to get waylaid by purple prose and side trips that don't go anywhere.
Of course there are exceptions to all these rules, but if you're new to flash, stick to the basics. Keep it as simple and on point as you can.