This was originally an email to a friend of mine who was asking for advice on how to write parts for the horn section in his funk band. I figure I spent way too much time writing it just to benefit one person, so hopefully someone else will find it useful as well. There is plenty of room for improvement, so please let me know if you have any suggestions, corrections, or contributions.
We're dealing with various issues here: basic part writing, orchestration (using the different instruments effectively and knowing their strengths and limitations), and stylistic concerns.
Let's start with orchestration because it's just good background knowledge:
The trumpet is in the key of Bb, sounding a whole step lower than written. (Forgive me if I'm telling you things you already know.) The trumpet's lowest written note is the F# below middle C, but the bottom octave or so has a much darker, less brilliant sound than the high range. Any decent player can hit a high C (two above middle C). A lead trumpet player in a funk band can be expected to hit Fs and Gs above that every now and then (and some professional players are even capable of another octave above that, but don't count on ever having one in your band). Playing in the upper range is very tiring, especially when the player is called upon to play long, sustained tones. Only top professional players can be expected to have the stamina to play a lot of sustained notes in the extreme high range. Bops are much easier, provided that there are sufficient rests between them.
I suggest entirely avoiding notes lower than middle C because the sound is pretty ugly and lacks presence. (Sometimes you might want ugly as a special effect, though.) The trumpet is capable of highly technical passages, but avoid them in the bottom octave where use of the third valve can get in the way. Glissandi are difficult on the trumpet; any slide of more than a half step would generally be performed as a half-valve slide where the player holds the second valve halfway open resulting in a less distinct, breathy tone quality. Double and triple tonguing are pretty easy on the trumpet, so you can write a single note in extremely fast succession with confidence.
The trombone has the exact same sounding range as a trumpet one octave down. It is written in C, sounding exactly as written. So the lowest normal note is two Es below middle C and the highest is roughly the Bb above middle C. The same stuff goes for the high range as the trumpet. Your player can probably hit high C or maybe even D. It is usually written in bass clef, although tenor trombone players with orchestral experience are often comfortable with tenor and alto clefs as well. Trombone pedal tones can be extremely effective. They are loud and obnoxious. They start at the third Bb below middle C and extend down, in theory, to the E below that, although the lower ones are progressively more difficult to play. Personally, I'm only comfortable down to pedal G.
Note that there is a gap in the trombone's range between the pedal tones and the normal range. Bass trombones have two thumb trigger valves which allow them to play all those notes quite comfortably. Many tenor trombones also have a single trigger which allows them to play all those notes reasonably well except for the low B, which can be faked but is probably best avoided.
The trombone's slide is sometimes a hindrance when trying to play fast running lines, although it isn't as much of a problem as most people think. In the upper half of the range, the instrument is actually quite agile. The slide also allows perfect glissandi within certain ranges. Each range is a tritone due to the length of the slide. They overlap more and more the higher you go:
The alto saxophone is in the key of Eb, sounding a major 6th lower than written. The basic written range begins at the Bb below middle C and extends to three Fs above middle C, though many players also have an F# key (ask your player). There is an additional "altissimo" range of notes extending another octave higher. This is that wonderful high, screaming sax sound that we all know and love. Ask your player what altissimo notes he is comfortable with; don't be surprised if stuff above written C (three above middle C) is a problem. The tenor saxophone has the exact same written range and fingerings but is in Bb, sounding a major 9th lower than written.
The bottom minor third or so of the playable range has a wonderful loud honking sound, especially on the tenor. Don't expect most players to be able to play those notes softly. The instrument is incredibly agile at playing runs, but isn't very good at rapidly repeating the same note. (You have to touch your tongue to the reed to start a new note, so double and triple tonguing don't work.) Don't write repeated notes faster than you can say "Ta ta ta ta".
The saxophone has a very distinct break in the middle of its range, right between the C# and D just over an octave above middle C. (Thanks to Haydn Lowe for letting me know the exact spot.) This is very similar to the break between falsetto and full voice, but is even more problematic for most sax players than it is for the average male singer. Making a big jump over the break with staccato notes is easy, but playing a loud, legato run over it is very difficult.
Saxophones have one feature which should make any singer or brass player jealous: the octave key. With this amazing device, the player can switch octaves as effortlessly as Bobby McFerrin. That can be a pretty cool effect which would be a lot more difficult to play on a brass instrument. By the way, if you haven't heard the Ani DiFranco track swing, check it out. It's a lesson and a half on how to write effective sax parts.
Both the saxophone and the brass instruments are capable of being played extremely loudly. They are the absolute best instruments at crescendo. Use this to your advantage. Forte-Piano-Crescendo is a very effective technique.
Part writing:
There are a bunch of guidelines for basic part writing:
There are a lot of other rules that don't apply to your situation.
Of course rules are made to be broken and these are broken very often. The thing to keep in mind is that you should save rule breaking for times when you want a special effect. Parallel fifths make a really cool sound that pops out at you (much like unisons do) and can be very effective, especially in a funk tune, but you don't want to use them all the time or they become hard to listen to and the effect is spoiled. Doubling the fifth of a chord is fine when you want that note to really jump out, but avoid doing it in the middle of a line where it would sound out of place.
Part writing is a skill that takes time to develop. I highly recommend doing some transcriptions of parts you like on recordings. Even if you have the parts memorized already, going through the activity of writing them down probably does more for your writing than any other exercise. Also try transcribing some melody lines and then writing your own harmonies to them.
Style:
Obviously you want to write parts that suit the style of music. You already have a good idea of what funk horn parts should sound like. You can further enhance your knowledge of the style by transcribing some of your favorite horn lines. Don't forget that unison lines are great.
In addition to knowing how things should sound, you need to know how your players are going to interpret what you write on the page. For example, if you write an isolated quarter note on a page and put it in front of an orchestral trumpet player, he will play a full note starting on one beat and ending on the next. If you put the same page in front of a funk player, he will give it a much harder attack and will cut the note short. In order to get the orchestral player to produce the same sound you would have to put an accent and a staccato mark over the note (and he still might not play it quite as short as the funk player).
I think these conventions date back to the swing era and have carried over into a lot of pop styles. They make it really easy to write the kind of parts you are trying to write without having to labor over the articulations, providing you know the rules to begin with. It's okay to break the rules now and then by explicitly marking different articulations, but the standard interpretation should be perfect 90% of the time.