There's a trick to aiming for a specific audience besides doing the market research and focusing on trends. It's a delicate balance requiring you to maintain heart but also attain marketability. The two do not necessarily coincide. Look at the big blockbuster movies. Many of them lack spirit. They are huge, loud and visually impressive without the benefit of feeling (outside of awe). Books can be this way too. Mass market paperbacks that hit the shelves of Target and Wal Mart can sometimes be the quick to press drivel that just sells copies.
Real success comes when you can take a public favorite (maybe it's vampires at that moment) and somehow instills it with some real heart. I tend to believe that people genuinely want quality entertainment (with a few notable exceptions of folks simply asking for fluff to get their mind off their day to day grind). Therefore, you should really try to put as much soul as you
can without sacrificing the material that makes it accessible to a wide audience.
Unfortunately, the way the publishing industry works, some trends that are 'right now' were actually written six months to a year ago. Your in the moment masterpiece may not be all that relevant by the time it's edited, covered, proofed and published. This is where anticipating the trends comes in and that can be really difficult. Who sets the trends? If you look at them and follow their work closely, even their blogs, you might be able to get up on them. This talent more than just about any other will
help you when you try to write for a specific group or for mass market appeal.
I personally don't bother. I write whatever the muse suggests and hope for the best. Maybe that's not the best plan and it definitely lacks a certain level of planning or strategy but when I get an idea, I'd rather run with it passionately than take a less exciting concept and put it together apathetically in hopes of a payday. Ultimately, I feel that we should be proud of every word we write and it would be hard to do that if we're compromising for a buck or two.
In the end, how you write and who you write for is entirely up to you. Ask yourself what you're in it for. Do you want a career that makes you cash? Focus on the above ideas and really strive to make your financial mark. Are you doing it for art? Throw caution to the wind and write whatever you want. When Bret Easton wrote American Psycho, he had a tight writer's group that all but ostracized him for the vulgar, ultraviolent nature of his book. He didn't back down from the controversy of it all and the book was very successful.
It can happen regardless of how you approach your work. Target and plan, write what you want, there's a lot of luck in this industry. Just do your best and follow your instincts. When you're done, you'll have something to be proud of under your belt because you did it your way. Your creation came to life through your own force of will and that's pretty cool.