It's hard to critique (in a good way) a piece that we haven't heard, but I can make some general suggestions. Is this roughly like a jazz band or stage band piece? You don't mention bass, but I generally don't write for drum kit without it. It's sometimes easier to write for a full band than for a smaller one, because each section is complete and easier to arrange.
First, write by ear! If you approach arranging as a part-writing problem, you'll end up frustrated because you're following all the rules but you don't like the sound. That's not to say you shouldn't master part-writing, but it should be used to work the bugs out of a piece after you've created it from a strong aural concept, rather than as a starting point.
Instead, close your eyes, think of each grouping, and imagine what you want each group to do. Think of it as 'figure' and 'ground', as in painting. What's going to separate the 'detail' and 'presence' from the pad? If you draw a very elaborate and detailed painting of a clown on a black or white background, the picture will pop. Paint the same picture on plaid or floral wallpaper, not so much.
Imagine what the grouping of trombone parts sounds like, what range they're in (in your head!), then imagine what texture will sound good against them. In general, if you voice dissimilar instruments, they sound better in unison. Also, when you have several dissimilar sections playing at once (like choir, low brass, and high brass) you can voice them all in one big tutti section, but if they're playing different parts it might turn to mud if you try to harmonize each individual section. Also, use some sections as rhythmic rather than harmonic content, playing little 'pops' rather than sustatained notes.
Most aspiring arrangers don't give themselves enough choices. For instance, unisons are a great tool that people sometimes underuse. As an example, a typical jazz band swing arrangement might have saxes playing the head in unison or octaves, the trombones playing the 'comping' part, with accented sustained chords, and maybe muted trumpets playing little stabs on the second time through. You're not trying to make each section do the same thing. Unison vocals sound good, too. A unison sound (choir, or trombones, or whatever) just playing a quiet 'guide tone' is a very nice and unobtrusive color.
In general, the faster the passage, the less harmony it will sustain. If you listen to the arrangements James Pankow did for Chicago, he's constantly changing those textures from unison to octaves to harmonies, right on the fly. Sometimes you can have everybody play the same thing, then fan out into harmonies. That approach was also heavily used in hard bop horn arrangements, and it is one of the things that gives the Thad and Mel band an agility that sounds simultaneously 'big-band' and 'small-band'. You hear the same technique in small classical ensembles.
In other words, don't forget your unisons.
Pay attention to note spacing. In general, the lower the notes, the farther apart they should be. I would typically voice notes at least a fourth apart below F in the bass clef staff. Higher up, you can jam things together. For instance, a full trombone section could play an Eb7 chord with a G (above the BC staff) on top, with the Eb and Db notes next to each other, and it would sound great, but if you put that down an octave it's going to sound very muddy even though all of the parts are in the normal range of the instrument.
'Hear' the parts in your head! Pay attention to texture and color! Pay attention to note spacing (the lower the pitch, the wider)! Don't forget your unisons!
I hope at least one of these helped you. Not sure where you're at on this road, so I might be telling you a bunch of stuff that seems painfully obvious. I guess the best advice is to start with silence, then try to improve upon it.