I just sit there and wait. Something comes eventually, usually at random times. I get these ideas and jot them down as best as I can. Sometimes I do crappy little drawings. I rarely have an exact method and I can't pump it out just like that. Right now, for example, I'm kind of hitting a bit of a period where I'm just lacking inspiration.
When that happens I'm just keeping on writing. There are lots of times where what I envisioned doesn't work out either. That's kind of why I just try to stick to big, core concepts. Too many details mean too many parts. More parts mean things are more prone to kark up.
I found it's quite frustrating when your perfect plan falls apart. That's why simpler is better. However, planning ahead with your fellow writers can really help out. Finding out what they want to do can be great for the story.
A lot of times bouncing my ideas off of them helps. They can help you come up with some critical points. They can also tell you when your idea is stupid and not feasible. In those times, don't be afraid to actually look at it critically because maybe it is.
Because your idea is dumb and it fails, doesn't mean that you're dumb and a failure. Ergo, be ready to change things a lot if it makes it work. Don't be afraid to look stupid because being afraid is counter to creativity. Also, write stuff down.
Because if your thoughts ramble like this post did, you might lose them.