Three steps to make sure you get more than 1 or 2 days out of it.
1.) Make it very crappy, and put it live for the world to see.
2.) Be embarrassed that this thing you made is so crappy.
3.) Use that embarrassment to make it better.
However, I suspect that the problem might be something else.
They say you should always be working on your best idea. Maybe you are jumping from project to project too fast?
Maybe you need to take more time to think about why you're starting certain projects.
Maybe the answer isn't finding more time working on projects. Maybe the answer is spending less time working on projects and more time thinking about possible projects.
Some random thoughts here. Sorry that it sounded a little Seth Godin-ish.