Here's a brainstorming sheet based upon mindmapping techniques. It is in .pdf file format.

http://www.libertyhallwriters.org/documents/LHBrainstormingSheet.pdf

This technique makes use of a trigger (aka a writing prompt). Triggers can come from something you saw or encountered during the day, from headlines in a newspaper, from another writer, just about anywhere. They can be sayings, images, songs, etc. It doesn't really matter what it is, just that it starts you writing.

To use this sheet, go to the central box marked "Trigger" and either write the trigger in there, or (in the event that the trigger is an image) write the first few things that came to mind when you saw the image.

Then, go to the left side of the page and start rapidly filling in a few items in each category, as quickly as you can. Don't think about it too much, or worry if what you write is silly, just do it.

Then look at your trigger again, go to the central categories below the trigger box, and fill some things in there.

Let all this swirl around in your head a bit, then go to the right side of the page and fill in a name for every character type there. Don't worry if you'll use them or not, that's for later.

Quickly scan the entire page so far, and then go back to the right side, and under each character's name, quickly fill in some distinguishing characteristics, be they physical (such as brown hair) or temperamental (such as prone to weeping).

Now, either throw all that out and go get a beer, or start writing.

Let me know how it works out for you, and how it can be improved.