1. Read all the time in your genre, but also read on a wide variety of subjects as well. Read magazines and newspapers. You'll be really surprised at the great ideas you will get just from reading articles in the paper, especially for mystery novels. There are tons of things going on all the time that could be turned into a novel. Keep your eyes and mind open.
2. Observe people. Notice how they speak. Watch adults and watch children. Watch their reactions to things, especially unexpected things, and see what they do. You might consider carrying a small notebook or voice recorder around with you to write down or record these observations. You'll be able to use a lot of it later in your novel.
3. When plotting your mystery, decide what the "twist" is going to be before you begin writing. What is the twist? That's the really interesting and surprising ending where your reader realizes that what he or she was lulled into believing throughout the novel wasn't actually what was going on after all. Some of the most popular mystery books have twists. But be careful not to throw out too many red herrings, as this will probably anger the reader.
4. Figure out your plot first, then add the characters. Your characters should arise from the plot itself.
5. If you are stuck for an idea for your novel, think about routine and mundane things that happen every day in your life, just those normal daily activities, and put an interesting twist on them. Like, for example, let's say that every night before you go to bed you check all the doors and windows but every morning your kitchen window is unlocked. Nothing is ever missing and no sign of forced entry, yet the window is mysteriously unlocked each and every day. Just that simple little thing is enough to grab your reader's interest.
You will not believe how quick and painless it can be to get that novel finished and ready to go to the publisher.