First of all, a disclaimer: this blog will follow what works for me. In fact, I may find that some of these methods DON'T even work for me, which will put me back at the drawing board. But I'll be sure to report in on what's working and what's not helping at all.
So here we are with the old blank page. It's good for brainstorming, at least. If you haven't gotten your idea for your mystery, this is the place to start a list. For me, it all starts with the murder. My last book featured a newcomer to a small town and a variety of people who disliked her for different reasons. Are there any news stories lately that have peaked your interest? Think of motives for murder: love gone wrong, greed, revenge, heat of the moment-type stuff. Just reading the newspaper (online or the paper variety) or watching the local news can give you ideas. You could change the outcome of the news story: ask yourself "What if THIS happened instead? What if it wasn't the jealous husband who killed her--maybe it was her lover instead. What if he were desperate to keep their affair hidden?" You get the idea.
It's also important from the start to determine what mystery genre you're writing in. These websites can help you: http://www.cluelass.com/guide/faqgenre.lasso http://www.booksnbytes.com/genres_mystery.html http://www.mysterynet.com/genres/
For example, if you're writing a cozy mystery, you won't want the murder depiction to be too graphic. The murder itself would happen offstage and usually isn't very gruesome. (Agatha Christie did have some fairly gruesome poisonings and stranglings, but usually cozies have lots of people with blunt force head trauma or have been pushed down staircases.) Determining what type of mystery you're writing helps you choose the murder plot.
I'm not going to spend a lot of time with finding the spark of an idea. That's one of those things that you just have to discover yourself. I picked up the idea for my new book while ruminating on dinner parties--a testament to the fact that you can find your ideas anywhere. I'm mostly interesting in the execution of the plot and the tools available to help it happen.
This is how I've started out the 3rd book--I've written the back cover copy. It does sound backward, but after reading this post yesterday, it sounded like a great idea to me: http://pubrants.blogspot.com/search/label/pitch%20blurbs . After all, no matter what your position is--aspiring writer, an established writer pitching the next book in your series, etc--you'll need a pithy pitch. Besides, she's right when she mentions it helps you to focus on the main plot catalyst. It also shows you whether your plot has enough punch and conflict. You can check out cover copy by walking through your bookstore, or online at Amazon. You might also check out your local library's website and see the subject blurbs they list for various mysteries. Choose your favorite mystery author and read his or her jacket copy.