The pre-writing work got me thinking about tools I've used for guiding brainstorming sessions. Basically, many people have no specific idea not for a lack of ideas, but for a lack of constraints on the multitude of possibilities.
One of the best tools is to introduce an external, not necessarily connected constraint. So, you can have a box of pictures (see
http://www.ideo.com/work/method-cards ) and you pull a card and try to write towards your topic in a way that you see as related to the picture on the card.
You can also use internal tools, probably the most common is "who, what, where, when" but there are other ways to constrain things. This sort of approach allows a logically inclined person to feel controlled in their exploration.
Building a set of imaginary fictional narrators and choosing one can also help to delocalize the proces, to distance the writer from the responsibility for their writing.
Writers' toolkits will have things like starter sentences, that can be good jumping off points. There are also fomulaic tools for getting thoughts out, kinda a more specific who, what, where with a Mad Lib structure.
On topic nonsense writing may also help, like:
"Christopher Columbus, third cousin thrice removed of the renowned Christopher Kringle, made a paper boat to sail across the bathtub."
