Writing Discpline

Discipline is a big part of working as a professional writer. It's important to focus on the task at hand, but I may take a short break to play with new ideas if I hit a lag and need to get the creative juices flowing. After my deadline is met, however, I will have plenty of ideas waiting. How do I decide which to pursue next? I consider two things: exhilaration and marketing. With which of these ideas am I most intrigued? Which will maintain my interest during all the stages of writing–including repeatedly revising and polishing? Once I've selected the three most promising, I'll consider markets. This is especially important for nonfiction. If it won't sell, why spend the time  working on it? Of course, trends change and an idea that may not interest an editor today may interest him in 2-3 months or 2-3 years.