Wow - this is one that I think a lot of people should consider. I've met a lot of writers out there that have their ONE BIG IDEA that they want to write. Its either a series or some big novel and that's awesome. To make it as a writer though, you've got to be able to keep coming up with ideas. And sometimes its with little to no notice. I'm not talking about the weird party trick you suddenly have to do of "telling a story" with little to no prep, I'm talking about shooting the shit with other people talking about collaborations, or trying to come up with new short stories or pitches for people.
1) Start Brainstorming Times
Its just like trying to work-out, start running, or even just writing in general. Its like a muscle and you've got to set up times to work on it. Taking time every day might be more than you need, but its definitely a practice that you should have to regularly work on being creative, asking yourself the right questions, and just generally coming up with new and weird ideas. Its a skill unto itself, and the only way you'll get better at it is by working on it.
2) Try New Things
Most of us are writers because we aren't great at getting out there, a lot of us (myself included) found comfort in books. Its easier to understand characters in a book than other people, and when they disappoint you they are much easier to replace than real people. If you don't try new things, read new ideas and bring other stuff into that head space you don't have as much stuff to try to turn into new ideas. You've got to keep pushing yourself outside of the safety zone and into that other place. I think there's a song about it.
3) Record, Record, Record
Oh man, this one has saved my bacon a few times. When you have those days where you're feeling creative and it feels like sparks are flying off your brain, each and every one capable of becoming a firestorm of creativity and great ideas - write those extra ideas down. Put them to paper. This serves two purposes - you can come back and figure out if they were actually any good, and on the days when you aren't creative enough to feel like you can change the world, you have this back store of ideas. Those are the days you can pick through it and grab the gems that you cleverly invested. Or you can just read through it and feel good about how amazing you are.