What I usually do is I take an idea I already worked with before and flush out all sorts of detail behind it, like an environment, location, or device of some sort, and just go nuts with it, trying to make sense of them. It'll start off as a few small things, but then you link them amongst itself and other related elements, and then evolve outward from there.
Like for instance, I start off with an idea of a floating island, I then come up with a reason (why) it's floating, and then what kind of people wanted to make it float; it then evolves into the kind of society and cultures spread throughout it, the architecture, the politics within, the unique technology, and it's reclusive nature; and then life within the island as a citizen, the fact that this island doesn't always stay in the same spot or face the same direction every day. There's an uncannily infinite source of water, only to discover that some of the waterfalls coming off of the island are actually going inside and are being filtered within the island and coming out fresh; power plants double as temples spread throughout the massive island community and united together inspire the community to practice a natural magic that comes through a complete understanding of how God thinks and asking for His permission fully aware the consequences that can form by even the simplest of actions. Toss in a couple adventuring explorers to visit the island and tell this crazy-ass place from their perspective and describe it again except as the adventurers, and forget and try to remember the details, and there's your story.
I remember I did that for a high school and a more elaborate illustrated version for a college course (told in a different perspective) with sketchbooks filed to the brim with ideas related to it, and the massive uni-/multi-verse/realities all this lives within. I got insane scores, I even hit the highest mark my school has ever seen regarding senior projects with a 198/200 score (then again, much of it was also actual research and public speaking stuff). I mean, an old concept of making a total melting pot of ideas and refining it into my own breed made me come up with all sorts of crap. a decade of working with this basic concept, and I'm still churning out all sorts of crazy ideas. Hell, some of them makes the show Ancient Aliens on History Channel far more interesting. How? I had all sorts of ancient alien ideas of my own, even as far as ships made of stone similar to, let's say, a sky island (or island cluster) the size of Hawaii that can transcend time/space like a TARDIS. Yeah...
But that's just my way of doing things. I never actually produced it. Had too much fun coming up with ideas and finding ways to make some of them real, mostly because they just looked cool, or they were an interesting project to undertake. Hell, I even had some ideas formed of perpetual generators; and screw hyperspace travel, foldspace is far more reliable and definitely quicker. Hell, use quantum physics to your advantage, and you have neigh-indestructible shields, or co-exist a dimension above reality, and refract matter through you and not get hurt one bit, or a dimension below and be entirely intangible. Think ORZ. Do so, and you'll be a happy camper.
Mayhaps evolving some of my ideas to some realistic parameters has knocked a few screws loose, but it is rather entertaining watching Lost and Fringe and actually managing to think ahead of the shows and their audiences (namely my family members) and hear/see everyone's reactions. It's as if I experienced them before, or at least bore witnessed and pre-deconstructed them in a past or future life, that's how far I got into detail with my ideas. It got to the level of being second nature to me. Incidentally, I nicknamed the whole -verse I would've based my stories off of (or my "Head Space") as 'The Second Nature'. Rather fitting and flows out smoothly. Could be a source of some of my bizarre dreams as well.
Oh, and pardon the wall of text there.
And with that mega-flashback that suddenly blindsided me, I am now really happy, and that made up for my crappy, yet oddly slow work day (the end sucked big time, it's explained in SAD). So to put it, some gears in my head have (FINALLY, after a couple years) started moving again and an entire keychain of memories that unlock archives of memories are being unlocked. I may be a little "off" for the next few days. Don't mind me, I'm probably going to be manically creative for a good while. Insanity will be expected. And if I make an art thread for myself and my work, I might share some of my illustrations. Now I just need some ideas and a good amount of time to render them. It's been too long since I last picked up a pen/pencil intended on doing something creative. However, I make something equal to my old work quality (or better), I think my old creative self might be back to stay again. I missed being creative for creativity's sake, not to gain popularity or earn money or anything. Being creative for fun (or even out of boredom) is a great experience. I missed it like an old friend I haven't seen for ages. I think that's why I liked the Myst Series (especially Uru) so much. It reminded me very much of my Second Nature.
And considering my new work path I lined up for myself, hopefully I can keep it intact and tend my creativity creature back to health and re-tame it to something I can use reliably again, and this time, protect it from criticism and encouragement to be used for things it's not designed for (like being sold as a book or something). It's like my inner child has returned and reminded me what I originally wanted to be since way back when. An inventor and an adventurer. Quite an odd experience getting your adult-self's ass kicked by your inner child-self. Adventuring may not be much nowadays, but I can still come up with some nifty ideas.
IDEA:
If it's a subject you need to start, you can always use Translation Party.
SUB-NOTE:
Writing my stories for classes aside, I actually illustrate a majority of my past ideas and re-read via flashbacks. ... Don't look at me like that.