The following guidelines affirm some basic aspects of how courtly and sacred art music moves from one moment to the next in the 18th-century. I have adapted all of these principles from Anton Reicha (Treatise on Melody: 1813), and Kent Kennan (Counterpoint: 1999).
1. Observe diatonic scale degree tendencies.
The least stable aspects of the major scale are the 4th and 7th scale-degree, because of their 1/2-step neighbor-relations to members of the tonic triad. Acknowledge their instability by complementing them with clear resolutions, usually fairly close to their occurrances.
In minor, the normal 6th scale-degree and raised leading tone are similarly unstable. None of these notes can be meaningful or graceful if it does not eventually find its way to the triadic note closest to it.
Chromatically modified notes (raised 6th in minor, raised 4th or lowered 7th in major) should likewise proceed clearly to the next note in the same direction (upward for raised notes, downward for lowered notes).
2. A step-skip (in the same direction) should be compensated.
“Compensation” means stepwise motion in the opposite direction of the the step-skip.
Compensation for skips and leaps of any kind is generally a good idea. In this special situation, however, it is practically mandatory in the style of Bach.
Step-skips should usually “arrive” at (i.e. the “skip” part lands at) a weak beat. This gives the compensation a greater sense of clarity of motion. When step-skips arrive on strong beats, their compensations will not be as effective.
3. Arpeggiation — two or more leaps in the same direction should always be treated carefully: the leaps must collectively form one harmony, and should usually be followed by a change in direction. Take care to create a good harmonic progression with arpeggios.
4. Sequences: any one of the above guidelines (or any rule, in fact) can lose its importance in a sequence.
If repetition is deliberate and clear, and reinforced by strong harmonic motion, then an unusual scale-degree motion, step-skip combination, or unfamiliar dissonance can become part of a larger pattern. Its coherence “stands in” (temporarily!) for the familiarity of the style.
Sequences depart from the norms of tonal progression, and consequently feel remote and unstable. (However, they are so common to us, that the instability is more a kind of mobile prettiness than any sort of longing or temporary discomfort associated with other instabilities.)and
They return to stability when they conclude with a cadence. In this sense, they are like large-scale resolutions of dissonance. Any of their unusual features, or remote harmonic statements, are balanced by the clarity of their motion toward an ultimate goal.