Voicing and balance
Voicing is how you spread a chord across your forces so it sounds full, clear and balanced. Give the melody the register and doubling it needs to stay on top. Space chords so they ring rather than muddy: wider at the bottom, closer as you climb. Decide what each moment needs and score for it, a tutti climax and an intimate verse call for very different densities. Silence and thinning are tools too; not every bar needs everyone.
Idiomatic, playable writing
Write for each instrument as that instrument, not as notes on a grid. Respect what sits naturally under the hand and the breath: bowable string patterns, playable wind lines with room to breathe, brass writing that allows for rest and recovery. Idiomatic writing sounds better and rehearses faster, because players spend their energy on musicianship rather than fighting the part.
Range, key and transposition
Choose keys that serve the singers and the instruments together, not just the original. A semitone either way can move a choral line out of a tired range or let a string section use open strings for warmth. Keep every part inside a comfortable working range for the standard you are writing for, and handle transposing instruments correctly so the sounding result is what you intend.
What helps: play or sing through each line as if you were the person on that stand. If it is awkward for you to imagine performing, it will be awkward on the night.
Part of The Orchestrator’s Craft Guide.