It's snowing to beat hell here (thanks for the euphemism, dad) and I'm on a week away from my day gig, chipping away at some BIG arranging projects.
The music on my desktop right now is about 700 measures and it often seems overwhelming. The original score material has millions of notes (that may not be hyperbole), but once you peel away the embellishment (i.e. orchestral color and flourish), any given moment in the score is just a few ideas.
I have a few thoughts that hopefully will be helpful to any who read this blog.
Writing a big arrangement is like any writing project; it's made up of hundreds (or thousands?) of smaller ideas. Good writing is a matter of polishing ideas (spelling, grammar, logical flow), and organizing them into larger blocks and then making certain the entire project has good flow and that your ideas remain consistent.
First - preserve iconic moments. The original is an orchestral score, and there are certain brass moments that should be preserved.
Second - work on one thing at a time. The music in the original is just a collection of notes. Decide what's important and keep everything else out of the way. Work on a passage and then move on to the next.
Third - write idiomatically, always. Harp glissandi, string tremolos, and piccolo chirping (to name but a few) don't translate well directly to brass. Why bother? Find another solution.
Sidenote - there are a few other brass arrangements of this piece of which I am aware. In each case, the arranger did a pretty good job of synthesizing notes into a serviceable brass chart. But I can point to passages in each version where, in the zeal to get as much of the orchestra score into the brass arrangement as possible, there is some writing that is not idiomatic. In other words, in those moments the arrangement does not give the brass group a reasonable chance of sounding good.
Too many notes.
Fourth - piccolo trumpet is a color, not a bucket in which to put all the high notes. This same comment can be made about the extreme registers of any brass instrument. In brass writing, two things can get banal really fast - writing all the bass notes in the tuba part, and screaming high piccolo trumpet as a default (pet peeves, in case you can't tell).
Finally - nothing is etched in stone. Once you have a collection of passages more or less polished, work on your transitions. You may have to redesign some of your orchestration at the beginnings or end of passages.