Call me strange, but it's a question I've never had to wrestle with myself. Ideas are all around us all the time. Many times they just appear out of thin air (that's one that no one can teach you, it just happens when you're in the shower or maybe reading a magazine or taking a walk).
"A writer never has a vacation. For a writer life consists of either writing or thinking about writing." Eugene Ionesco
He pretty much had it right. And, it's the 'thinking about writing' that keeps you open to new ideas as you cruise through life.
Think about it. No matter where you go, what you see, there can be a story behind hit. A baseball game? A player gets slammed with a line drive ball, knocked unconscious. I saw it happen. But then, a woman jumps over the wall down into the field and runs to the fallen player. Girlfriend? Wife? Sister? Fan? Is he seriously injured? Could he die? See a story emerging?
And what about the hummingbird you see hanging right outside your window, wings going like mad, suspended, seeming to stare through the glass at you?
Is it a sign? A spiritual experience? Just a hummingbird, but is there something else coming? Can you connect it with something like a "Narnian" story? Fantasy or Science Fiction perhaps?
To keep your ready source of ideas flowing you need to teach yourself to be aware of your surroundings; to watch people; to observe nature; to read newspapers and magazines with an open mind that may pluck a good story idea from a few sentences.
Bears come down from the mountains into the city in great numbers due to a fire destroying their homes and food. What now? Is it a great nature story with a hero saving bears and people from each other? Or does it morph into a supernatural story of bears gleaning a hive-like intelligence after the disaster and lumbering into town bent on revenge for their loss?
Ideas can come from friends and acquaintances as well. Stories they tell you. Things they see happen around them and can't wait to tell someone else - you.
Many ideas won't work out, but you have to play with them, think out the 'what ifs' and you'll find yourself spinning tales daily.
"Life is what happens to a writer between drafts."
Damon (aka Dennis R. Miller) …who spent 25 years completing his novel The Perfect Song.