i have a very tough time coming up with ideas but generally once i snag on one, i stick with it. i can usually suss out the ideas that have something worthwhile (to me, anyways) at their core, so if i implement it and it doesnt work out i just iterate on it. and if i dont like that one i just iterate on it again, and so on and so on....
its not even a matter of prototyping, really (which might be a bad thing). generally i have an idea of the intersection between the kinds of games i want to make, and the kinds of games i know HOW to make, and if the idea is too far outside that intersection in either direction i abandon it midway through figuring out the design.
and then if i get thru the design phase, i start prototyping, which is where a lot of flaws reveal themselves. usually i dont even finish making the prototype before i realize whats wrong with the idea. but if im still working on it for 2-3 days without finding flaws or beginning to hate the design, i usually stick with that game.