I'm often reminded of an old saying: There are no bad ideas in brainstorming.
This is a lie.
There are plenty of bad ideas. Take a walk through the "state-to-VHS" section of your local -mart store to understand. I mean, have you even seen the last fifteen Hellraiser movies?
But brainstorming is a critical part of our profession. If you want to be a writer, and actually write something now and again, you need to be in a constant state of creation. It's no easy task. There are days where you may simply not want to write or create.
Remember, this isn't your hobby. If you want to be a writer, and actually make a living wage, you need to treat this like a real job. Because it is actually a real job. It takes work.
What I recommend is this: Go out and buy a notebook. Buy three, actually. Put one next to your bed, one next to the toilet, and one in your pocket. Wherever you, and whenever an idea strikes, write that shit down. I don't care if it's the lamest, most basic concept ever conceived. You need to capture that thought for later review. Even if you only come up with one idea a day, you're still creating. You're still opening yourself up to the chance of success.
And so, every single day, you need to be creating new ideas. But here comes the hard part...
Some of them will suck.
It's the honest truth, and that can be hard to hear. You are going to come up with bad ideas. You will blatantly plagiarize existing works, only to realize it halfway through your first draft (my original take on The Gray Wars opened with an Independence Day rip-off). You will come up with the same idea twenty or so times, only to forget it at the end of each session (a look through my notebooks shows a predilection toward zombie fiction, all with the same main character). This is all okay.
But you need to persist. You need to come up with ideas constantly. And that means you need to accept your bad with the good. In fact, and here is where I go back on an earlier statement, you need to believe that the ARE NO BAD IDEAS (even though there are. Please see "Fant4stic Four").
Because, when it comes to writing, there is only one bad idea: Not doing anything.