The problem is that when focus is shifted to a single idea the remaining ideas still seem intriguing and they lure me away as the rough patches hit during the current project.
Sometimes you have to scratch the itch in order to focus. It might cost you a day or two, but if it helps you get back on track it is probably worth it rather than weeks of being unfocused. If you find the new thing you're working on so much more compelling, you might be better off sticking with it, especially if you're more likely to stay with it.
You might be interested in using a stage gate method for helping to determine whether or not to keep with the current project: http://lostgarden.com/2007/02/rockets-cars-and-gardens-visua...
I think it is especially hard to let go of failing projects and being able to put things on the shelf will save you years of lost time. Is the rough patch an indicator that maybe the idea isn't as good as you first thought or just hard work, or?
If I start a project however, I will focus only on it, until its finished (thats not to say it then receives no attention). Only then can I start looking at the other projects I want to do - and decide which to begin next.
Otherwise nothing would ever be accomplished.
startup cost
time to build a mock up
business model
estimated revenue in a year
passion index
competition
comments
then you'll know