Does this mean you should never write "off the cuff"? Funnily enough, no.
I have brainstorming sessions every so often which I find incredibly useful for producing potential ideas for future stories and blog posts. There is no planning or preparation for this whatsoever.
This is unlike everything else I write though. I do sketch out a structure for the articles I write. I have my beginning, middle and end in mind before I start writing.
I outline my stories (sometimes in lots of detail, others with "broad brush strokes" and yes I've done this for my flash fiction work too!).
I find this kind of preparation, whether it is for fiction or non-fiction, helps me produce more work, not less. I think it helps me write more efficiently when it comes to producing the actual piece. This blog post, for instance, I knew I would share something of how I work as a "peg" to hang the rest the piece from. And that to me seems a good place to end this other than to say comments on how you work, what preparation you find useful etc would be most welcome.
Have a good writing week!