If its a good idea, I write a page about it in a google doc. I try to capture the essence of the idea and why its exciting. Then I forget about it and get back to work.
If it's a really good idea, I can't forget about it. That's a good sign, but I get back to work anyways.
If the idea doesn't go away, then maybe it's time to spend a few extra hours exploring the idea and its feasibility. What would it take to create a rough prototype? How much would it cost? What are my resources? How would I fund it? How does it fit into the market place? How do I sell/pitch it to customers? How do I distribute it? Does the idea pass through all of these filters? Good, keep it cooking, but get back to work.
Okay, I made a really rough prototype. The idea isn't dumb. It works. How much time and effort do I want to dedicate to further developing this? Is it better than the current project? Is it more profitable to stop my current project and develop the new project? Can the idea wait for the current project to wrap up? Do I have a habit of leaving half finished projects laying around because I can't maintain the motivation and drive to finish them? If yes, then what's to say that the new exciting idea won't meet the same fate?
In my eyes, this is a ranked list of what's most important:
#1: Do I have a product customers will buy? How do you know? Are you sure? Really sure?
#2: How will I market my product to customers?
#3: How will I distribute my product to customers?
#4: How will I create the product that customers want?
#5: How will I manage the project and walk it through from beginning to completion?
My questions are customer oriented and project management focused. Ideas are a dime a dozen. They're essentially worthless. What matters is creating something tangible which you can sell. The creation process is making an idea happen -- executing. As many of you know, simply completing a game and releasing is not enough. Lots of great, polished games are released every day and still fail because nobody buys them. Find out why others fail, and don't do whatever they did to fail. You will probably still fail, but at least you didn't fail for the same reasons. Success probably happens not because you did everything right, but because you didn't do anything wrong.
So, when I think of an idea, I write it down and it has to pass all of my filters designed to help me avoid doing wrong things. Good ideas float to the top, bad ideas stay in the pile of ideas and probably never see the light of day.