Write what you love. And get lucky.
It's so true. Nobody knows what the market will reward, so you can't write to the market even if you could force yourself to do that and still produce good work. Sure, you can sell out, toss in a few gratiutious sex scenes, an unnecessary explosion, a syrupy moral or two - who hasn't done that? But in the end, you have to write what you love, and hope someday the market catches up to you.
So, here's my program for becoming a professional writer.
- Go to step 2.