This happens to me all the time. I'll get an idea for a new site, think of a catchy domain name, register it and then after a day or two I lose interest. Usually this happens when I choose topics that I am not as passionate about as I first thought I was. So now I make myself write an outline when I have a new idea and if I can't fill up the outline with enough ideas, I chunk it and go back to square one.
I'm working on a new site now that I hope to launch in a couple of weeks and it passed the outline test so we'll see what happens.
