Worldbuilding is a quandary for many writers. You want to craft something vivid and compelling, that doesn't show Tolkien or Asimov in the creases, that creates a space to support and enhance your story.  It's far too easy to get lost in the process, though - to be set on making a complex and detailed world, and forget to actually write the story that fits in it. But it's the story that often makes a world whole: without characters and emotional connections, it's a bunch of lists and maps, and of little interest to anyone but its creator. 

There are three broad approaches to world building. Traditionally, there are only two, actually - Top Down, and Bottom Up. But I think that cuts out a crucial method - the one that (in my opinion) is key to creating something vivid and original without forgetting its place as a backdrop. I've nicknamed it building Sideways.

Top Down: Universe to phage

Top Down building is starting from as broad a view as possible, and building down into specifics. Start with a view much broader than your story will encompass. If your story is contained by a household start with the village. If your story spans several planets, write the solar system, or galactic quadrant. Make the rules for the story's universe that all other worldbuilding must obey.

Then you work down to specifics - the planets, the continents, countries, cultures, people. There are a lot of things to consider, and I'll leave the details for another post, but Top Down world building can take a lot of time - a lot of time, and most of it spent building things you'll never end up using. If you're methodical, however, you'll end up with a logically consistent universe to play in, which is ideal if you're planning to write a series. The more stories you hope to set in that environment, the less you can afford to wallpaper the cracks, so for long running series, it's advisable to at least run your universe through a Top Down view, just to find the holes.

Bottom Up : Dark street with a torch

Building Bottom Up is essentially creating the world as you go, as it's needed. If your story starts in a household, you build that household, but no more. As young Jimmy walks outside into the village, you build the village - but only the bits Jimmy's looking at. The rest is concealed underneath the fog of I-don't-have-to-deal-with-that-yet. 

The problems, when compared to the Top Down method, are pretty obvious. Without overarching universe 'rules', you've got no real handle on consistency, no notion of whether a decision now is going to make a decision later really awkward, or impede the direction of the story down the road. While it does (or can, rather) get you writing right away, you also run the risk of a lot of rewriting when you discover things just aren't going to work the way you've laid them. For a small pieces, that likely doesn't matter - the scope of the world is narrow enough that you're unlikely to run into too many pitfalls, and those you do can probably be sidestepped or even lampshaded (that's a tvTropes link. You have been warned) without impinging on the story.

Sideways: Six degrees of separation

Building sideways is a way to marry the two. It gives the consistency of Top Down while avoiding the behemoth of building everything, and it lets you get to writing by building what you need right now. It works as a network of ideas - one idea has toeholds in another, which will effect another, and so on. 

You build whatever you want. Typically, I start with some major part of the story - that household, for example. I make the household. Then, it's time to explore: what's influencing the household to be that way - what are the broader ideas that lead to these people living this way? I dont need to detail them fully, just note them. What other things do these broader ideas influence? What things does this household itself influence?

The second step is - you build whatever interests you. You have a great idea for a plant-creature that stalks humans and burrows underground? Build that. Now, what environment would support a creature like that? What other creatures would that environment support? What things would that creature affect?

And the third: how do these two idea-collections you just built connect and affect each other?

In this way, the world building branches out as the story progresses, always a few steps ahead and around of where you're going. Thinking through the path of how's and why's and the interconnections gives you a greater (chance of) consistency and logic and, I find, often sparks a much more interesting path for the story to take.