Writing for whom?

As a journalist, in my day job, one of the first rules you learn is to REMEMBER YOUR READER. All publishing companies spend a lot of money researching what their readers like and trying to meet their demands. It's a lot tougher doing that, however, as a first time novelist. You know what you like and who you think you're writing for but it's tough to get a real picture of that person in your head.
I try to write the kind of thing I think I like to read. Anyone have any suggestions about how to focus the reader in your mind?