Back to the issue of the individual lines within a harmonic accompaniment. While you’re no doubt thinking of the way the notes stack “vertically,” the line itself is still played “horizontally.” To that end, it’s important to examine the melodic lines you create for each individual instrument or voice.
One way to create a more melodic harmony line is to follow general voice leading principles. Voice leading is what you get when you follow one harmony part from start to finish; the different intervals between the notes follow a set of conventions and act to create a pseudomelody out of the harmony line. You have to make sure that one note properly leads to the next, to avoid having the harmony line sound like a bunch of totally unconnected tones.
Move Smoothly: Most often, voices should move the shortest distance possible; small movements are easier to play and sing than are big leaps. How big of a leap is too much? In traditional harmonic writing, you rarely see inner voices leaping more than an octave. Mostly they move in step-wise motion, or by leaps of a third or fourth.
Move in Different Directions: When you’re writing individual parts, you should avoid moving all the voices in the same direction all the time. This sort of excessive parallel movement is lazy and can sound a little boring; if some of the voices move up over the course of a line, at least one voice should move down. Using this sort of contrary movement adds interest to the inner voices in an arrangement.
Avoid Crossing Lines: As much as you want to use contrary movement, you also want to avoid crossing lines in certain parts. Now, a certain amount of voice crossing is unavoidable, especially when one part is serving a primary melodic function. There’s nothing wrong with that— especially if the lines are in two distinct instruments or voices. But when lines cross between similar instruments or voices, then the implied harmony gets a little muddy. In particular, you want to avoid voice crossing within a section—by two trumpets, for example. And, when writing choral music, you also want to avoid crossing the bass and tenor lines, and the soprano and alto lines. (It’s okay to have the tenor and alto lines cross, however, since the timbre of these two voices is noticeably different.)
Emphasize Common Tones: Another popular technique is to identify those notes that are the same from one chord to another—what we call common tones. By emphasizing the common tones between chords, you can better connect one chord to the next within a given part. When a given voice holds the same note across two (or more) chords, it creates a powerful bridge between the two chords.
Let Leading Tones Lead: Whenever possible, you should let voices that sit on the leading tone within a scale move to the natural resolving tone—the tonic of the scale. When you have a vocal or instrumental line that’s holding on a leading tone, the next natural movement for that line is up to the tonic note of the scale. While you don’t have to move this line up to the tonic, it sounds good when you do.
Focus on the Bass Line: It’s only natural to pay attention to the parts at the top of the pile—and to place the melody on top of all the other parts. But the parts on the bottom are equally important and can often provide an interesting counterpoint to the top lines. To this end, pay particular attention to your piece’s bass line; it doesn’t have to simply follow your chord progression. A rhythmically and melodically interesting bass line can be the difference between an average and an exceptional composition.