The proper way to get ideas, of course, is from the mail order firm in Schenectady. It offers five-idea, ten-idea, and the economy sized twenty-five ideas packages. . . and the real problem with telling that story is that if you use it, people will ask you for the address.
But, for those of us lacking the address, the first rule is that all is grist. Therefore it behooves you to get your hands on stuff. Read stuff. Lots of stuff. Read fiction in your genre and out of your genre and in public domain. (Bad stuff is, in fact, particularly useful, because you find ideas that were not deployed for their full potential.) Read history, primary and secondary sources. Read science. Read news. If something's particularly apt for your genre -- such as science for SF -- you may want to concentrate on it, but not exclusively. You never know what might prove useful. And play attention to just about everything, because what often helps is a random fillip of information that sets your imagination running. Or something you misunderstand and try to rationalize into a story.
A notebook you can easily lug about is a good idea. Ideas have a peculiar delight in popping up in situations where it is difficult to memorialize them.
Once you have an idea, the first thing that you should consider is whether it needs pruning. This is particularly important with cool ideas from copyrighted works, but anything where you have an existing story can be dangerous, even public domain -- even real life. Because when you run away from a story with an idea tucked under your arm, you may have taken a lot more than the essence of what interested you. When telling a story of a poor orphaned boy who lives with his aunt and uncle and cousin, who maltreat him -- does he have to be an orphan? Is that what really interests you? Unless you want the story to call back to the other story (only really safe with public domain or parody), filing off the serial numbers is necessary.
But even if it doesn't need pruning, the idea will doubtlessly need growing. The first question to ask: what does this idea entail? If your hero is cursed and brooding, what is the curse? Why was it laid on him? What happened to him to make him brood over it -- or is he just naturally melancholy? Or if you have a neat idea for a rescue from a magical prison -- a good idea in itself, since it holds conflict -- who is the prisoner? Why is he imprisoned? Is he guilty or innocent?
Pruning the idea in fact relates to that, because when you see what you can prune, you are seeing what certain aspects of it entail for the story.
If the idea doesn't contain conflict, it needs to be introduced to some, and so the question is: how can it cause conflict? A cool MacGuffin needs a character to be passionately attached to it, as a symbol, as a trophy, as a vital piece of magic -- and another character to be either as passionately attached, or to have powerful motive to hurt the first and the knowledge that the MacGuffin is the trick. And if you do have conflict, you need to have characters to which it will be significant. So another trick is to ask who would find this very, very, very meaningful? (There's a reason why most military stories start with a raw recruit.)
Sometimes story ideas grow into full stories. On the other hand, sometimes it helps to run a dating service. What would this intriguing character do with that problem, or this magic item? Ideas that refuse to grow may, on be considered with another, crystallize into stories. And for novels, steadily introducing story ideas may be the only way to actually work out to novel-length; few story ideas can bear out a full-length novel without any support.
What are some of your tricks for catching and caring for ideas?
And what is the weird way you've ever had an idea strike you?