Coming up with ideas has always been a problem for me - always - and I don't think it gets any easier with time. At a reading of my story Roar Shack, the other writers in attendance and I were asked where our particular stories came from. In the case of Roar Shack, it was basically an assignment. The monster theme came from Penumbra in a list of themes they were covering before they closed their doors.
That's the easy way to get an ideas, but there are others.
Every story begins with an idea, and to have any hope of being a productive and prolific writer, you need to keep the ideas coming. I'm speaking for myself, of course, but I think an hour a day at least a few times a week should be spent on brainstorming. Even if you're busy with a bunch of projects, I'd rather have something rather than nothing.
Here are some things you can use for your own brainstorming sessions...
Get out of the office.
This is something I heard from independent filmmaker Ryan Connolly who hosts an entertaining and informative Youtube program called Film Riot. Couch time is exactly what it sounds like: you stretch out on a couch running ideas through your head. Shark Tank's Barbara Corcoran says, "Go outside. All the big ideas are on the outside. You'll never have a creative idea at your desk." The bottom line is you need to get out of the office. You might not come up with your next story in this brainstorming session or that, but by getting away from the desk, you're telling Serendipity you're available.
Notebooks are good.
Stephen King says notebooks are the best way to preserve bad ideas, but I also suspect he's got a psychic connection to Todash Space; he might not be willing to admit it. King says that everyone's method is different, so while the notebook might not work for him, it could work for you. I keep one of those composition books on hand and just jot down whatever comes to mind, especially when I'm working through particularly difficult brainstorming problems. No one might read it. Even I might not read it. But writing things down has always been how my brain works. I kept about a dozen journals in college, never looked back on most of them, but I can still remember a few good ideas that bubbled up. I also keep a little pocket notebook in my pocket when I'm out and about. If I'm working on my MacBook Pro and something comes to mind, I have got the stickies app to make a quick note of it.
Talk to friends (real or imaginary).
An idea might sound good on paper, but it could fall apart when spoken out loud. If I have a chance to brainstorm with a close friend of mine, I'll do it. It's hard to get out of your own head, so having someone else with you is a good way to get a new perspective on ideas. I hung out with a friend of mine the other day and bounced ideas off of her on an alien invasion story (something I've wanted to do for a long, long time). A lot of the ideas I had for specific story elements had been in my head for a few years. At the very least, it was a chance for me to ask, "Does that make sense?" It could make all the sense in the world to you, but bouncing ideas gives you the audience perspective on it. If you don't have someone to talk to, think out loud. Tell your idea to your houseplants, and then be the devil's advocate. Sure, people will think you're crazy, but if they know you're a writer, they'll probably think you're crazy anyways.
Put on some music.
Along with couch time, it's hard to think of listening to music as part of the workday (unless you're in the music industry). Not only does a particular song help set the tone you might be aiming for, but every now and then, your ears might pick up on a particular line in the lyrics that could stir an idea.
Some imagination goes a long way.
When all else fails, there is one instance where you could brainstorm in your office: imagine you're in a writers' room. I used this when I began work on my screenplay after taking a TV writing course on building stories and outlines. I also worked at an entertainment company at the time. Basically, I imagined my boss calling me up one morning saying, "Mario, Studio X wants a story about Theme Y. Think you can come up with something?" Theme Y is whichever topic I'd like to write about and gives me a starting point. Then I come up with ten one-line ideas revolving around it. I develop the most promising half into a short paragraph, and then pick the most promising of them. I go right into drafting from that. If it becomes a novel, great. A short story is the least I can hope for, but one can dream. I'll turn the idea into a page-long synopsis and then and outline if I'm working on a screenplay, but only because I've learned writing for the screen is a different animal than for the page.