science fiction isn't as easy as it looks. here's are rules and guidelines I follow (loosely):
Heinlein's Rules of Writing
1: You must write.You can't write a villianous overlord without looking through Peter's Evil Overlord List. Avoid the mistakes everyone else makes when writing villains (the ones with henchmen). This site gives such sage advice as "shooting is NOT too good for my enemies," and "no matter how well it would perform, I will never construct any sort of machinery which is completely indestructible except for one small and virtually inaccessible vulnerable spot."
2: You must finish what you start.
3: You must refrain from rewriting, except to editorial order.
4: You must put your story on the market.
5: You must keep your story on the market until it is sold.
6: You must start working on something else.
There's some great advice at Nick Lowe's The Well-Tempered Plot Device.
If you're making a fantasy world, consider putting yourself through the paces at Patricia Wrede's Fantasy Worldbuilding Questionnaire. Worldbuilding is a fun thing to do - but, in order to make your story believable, you have to have a natural base upon which to build your system of magic. You have to have an evolutionary history for your Deemogs. You have to have a reason why your hero says "Geehig" instead of "Hi" when greeting the King of Dragonholt. They have to have religion, medicine, transportation. A good example of this is Tolkien's Lord of the Rings: possibly the most detailed world built in the annals of fantasy.
Here's a similar worldbuilding device for Science Fiction, by S. Andrew Swann. Worldbuilding is even more important for science fiction - because it's the physics, chemistry, culture, etc., of that world that is absolutely, completely integral for your story.
Research. Research. Research. Some sites that can help the iggerant layperson:Tips on Writing Military SFAnd, finally: the essay that, many years ago, helped me flesh out my characters. Ladies and gents, James Patrick Kelly's You And Your Characters.
Writing SF for Kids
Writing Soft SF
Putting Science in Science Fiction