i have hundreds of unfinished games, because i usually cut my losses early on, and i'm okay with that.
first stage, the idea, it's usually spontaneous. i work it out in my head til it sounds presentable, and if not i just give up on it.
second stage, i pitch it out the other people near me to see if there's a spark of hype, if not, then i stop there. however if i feel strongly enough about it, i try to prove the concept with a physical prototype.
third stage, if the prototype doesn't prove to be fun after a ton of iterations, i scrap it, and if it does, then i start brain storming for bigger development, getting an idea of how much work is required and if it's compatible with my current resources, such as time, patience, skills, tools, experience. i give up if it's just too ambitious.
fourth stage, i feel good about it, i begin the digital prototyping, flesh out some concepts. this stage usually lasts about a week or so before i can come to the conclusion if i really want to commit. i never force myself to commit to something i don't believe in anymore, it feels like wasting precious time. i pay my respects to that, the experience has made me better, then let it die.
fifth stage, currently at that with my current project which i've been working several months on, i can't sleep without thinking about it, i can't stop not wanting to work on it. i am obsessed with it. and i'm actually passionate about it. this is my goal and where i want to be.
so essentially, if my brain doesn't want to focus, i quit. because i trust my brain. i have no idea if that method is practical, seeing that i never finished game. but i'm okay with that.