I usually do short little drabbles or write out a scene when I get certain ideas. (About a quarter of my oneshots are literally taken from short drabbles that I've written and came back too a couple of weeks/days later, able to expand on things and actually write them.) Or I just write the whole idea out with different possibilities and if I still find that I want to write it weeks later, then I might write it. (I get so many ideas, but only like a tenth of them actually come to fruition and even then, it's a toss up if I can actually finish them sometimes.)

Talking out your ideas with a friend is also good to do, since it helps you clear up an inconsistencies with your idea as well as possibly fleshing out things you didn't or probably weren't going to flesh out because you didn't think about it.

Or you could possibly write it out, come back to it a few days later, and rewrite it with a clearer head that can focus on writing things well, instead of just trying to get the words out.