Sorry to come so late into this thread, but I don't often come to this forum (Platform & Stage).
Historically, when it comes to deriving a script, I usually do it developmentally. That is, I start with the routine (scripting the effects), and then I start rehearsing it sans patter (to get a sense for the visual flow). I'll videotape the routine (but I am performing silently). It's only when I feel comfortable with the structure of the routine that I start formalising what I will be saying when I perform the routine. However, by this time, the ideas for patter (what to say and when) are in the back of my mind. Now I start performing the routine with patter (to the video)... So, I never actually sit down and write the patter per se.
However, sometimes a routine starts with a premise (and MY best routines probably fall into this category). That is, the premise for the presentation predates the routine. In that case, the patter comes first. Here, I am trying to tell a story (in the figurative, not literal sense), and I am now trying to think what would happen next, or what should I do next (if the premise were true). In this case, the patter comes first. In these instances, I tend to sketch the patter - it's not a word for word, more an outline.
So, historically, I never wrote down the words for my routines; the patter evolved through repeated performances. However, I read Ken Weber's Maximum Entertainment last year, and one of his top recommendations was to write down the patter (word for word) for every routine, and then to edit it so that every word served a purpose (ie, there is no filler).
First off, this wasn't easy, as I had to type it all up (a five minute routine takes about three letter sized pages). But the funny thing is that having done this, the patter then develops in leaps and bounds. This is because you are forced to think about what to say at every point in the routine. Obvious comments like "I take the ball and place it..." sound so lame when written up, so you have to think of something better/different. But, then you have to LEARN the patter. Rafeal Benatar talked about this recently in Genii. One of his recommendations was to learn the patter backwards (ie, on a 3 page script, learn page 3 first). Otherwise, you always start at the beginning, and learn that bit best.
Phew... I've gone on long enough
Bye for now