It's a fairly common experience: an idea pops into your head, usually out of nowhere and without your conscious knowledge.
In reality, your brain probably had a problem it was trying to solve. As you wandered through your day, your imagination came into contact with some piece of random stimulus, and intrigued, it merged the problem with this new piece of (something),and suddenly you had a new idea. I'm a visual person, so for me, it's usually an image, something I see, a picture in a magazine. Other times it's a piece of music or a lyric. Maybe you're the type who prefers words or reading? Or you're tactile?
Whatever it is, this simple trick of merging the problem with a random concept is the easiest way you can create an idea. If you want to do this more often, then it's simply a matter of creating a series of random images to keep on hand to spark your creative streak.
Whenever I need to brainstorm an idea, either by myself or with a group of people, I pull out different sets of 'flash cards' that I've made or collected over the years. I/We ruffle through them, allowing our imagination to use the visual and verbal stimulation to generate ideas. Next thing I know, my imagination is pouring out ideas - both good and bad, which I can then adapt, adjust, continue to re-merge, until I have a few good potential ideas.
How to get started ... there's several options. The cheapest way is to collect free postcards. (See top picture.) I can't speak for the rest of the world, but every coffee shop in Australia has a rack of free postcards, most of them visually rich. I take one of each and add them to my collection ... which is about 500 cards by now, I think. I've also started to buy postcards of artwork at museum gift shops or ones of tourist spots in airport news agents.
A slightly more expensive option is flipping through magazines and newspapers. In fact, I keep a bin of old magazines next to my desk for this reason. (Second photo.) Or, rip out images and put them in a folder you can slip out of your desk drawer for instant brainstorms. (Third photo.) As you can see, I prefer visual images without words, but choose anything that stimulates you.
The most elaborate method is to create images on white cardstock, then laminate them like professional flash cards. (Fourth photo.) I still have a set made for me when I was a creative director in New York, and while expensive to make, they're indestructible ... and strangely impressive to clients. Alas.
Maybe you're not a visual person? Then use a dictionary or thesaurus. Choose a random word from any page, and it'll stimulate the imagination in the same way.
Finally, open Google, Yahoo, Flickr or similar. Type a random word into the image search engine, and off you go. Instant flash cards!
If you have any of your own suggestions, please send them here. I'll post each individually in the future, and give you full credit.