I love SF, and some of my favorite stories are about other worlds, including Andre Norton's adventure-filled books, Christopher Anvil's Pandora's Planet, Gordon R. Dickson's The Outposter, Anne McCaffrey's Pern series, Robert A. Heinlein's Podkayne of Mars, and Keith Laumer's Retief series.
Usually I start out with a story and then fit in the background--planet and culture, though some of my stories are set completely on Earth. While my main characters are usually Americans in stories set on Earth, elsewhere I often give Terrans backgrounds of other countries and cultures to make them more interesting.
Some planets are simple, with little description of wildlife, moon(s), etc. (I don't want to worry abo
ut tides and how things evolved). I've spent more time inventing planets like Snakebite in Hidebound, which also included the hero's planet (one even nastier than Snakebite so that the humanoids evolved physical protection); I made this planet interdicted. And then there's the colonized planet in Velvet of Swords (more nasty flora and fauna as the result of genetic engineering). It was colonized by humans and aliens, with the humans indulging themselves in old Terran cultures.
Other interesting planets are found in What Price a Friendly Freep to explain the aliens; Pretty Pink Planet and Hot Yellow Planet, which was began, as I recall, as an experiment in writing a series story with similar titles; Royal Guardians (I think this is an alternate universe); and When Danger Rules, where the royal family is strange. Time portals from Terra to other planets or time machines to other times are fun too.
There are books (see Writer's Digest Book Club listings) and websites, such as Patricia Wrede's Worldbuilder Questions: "
http://www.io.com/~eighner/world_builder/world_builder_index.html" , on world-building; I haven't spent much time there, but I've discussed various ideas on AOL writing boards, where a writer can ask for input when trying to solve a story problem. I recently asked about missiles and subs in the Zap Gun folder (SF/Fantasy board), where we also discussed Keith Laumer's Bolo (super tanks) series.
For some stories, I've had to create maps to keep track of where my characters are running amuck. I have to keep track of directions and distances. If you're writing a story about Mars or the moon, however, you can use NASA maps, available in books, on websites, or even as posters. There are also Mars and moon globes. Nowadays, there is less invention in stories set there.
So, you can find the blocks for building your world in the far corners of the universe of the mind, but for decorating and landscaping, you may want to research other planets and other cultures (I think the Celtic culture is way over-used, though that's mostly in fantasy), found in fiction and non-fiction books; then you can put your own twist on a planet, an animal, or an intelligent being. And don't hesitate to use a tentacled alien; they're not passe if you can add something new.
JoyJoy V. Smith has been writing stories since she was a kid. Her stories have been published in print magazines, webzines, and anthologies; and her SF has been published in two audiobooks, including Sugar Time. Her recent non-fiction includes her book, Building a Cool House for Hot Times without Scorching the Pocketbook and an interview with Lyn McConchie. She lives in Florida on a registered backyard wildlife habitat with Xena the Warrior Puppy.