The_Complete_Idiot%27s_Guide_To_Music_Theory

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Part 5: Embellishing
Avoid extended parallel movement.
Note that this approach created eighth notes in the top part when the bottom
part is using quarter notes, and vice versa. This provides a simple rhythmic variation
without going whole-hog on complex interrelated rhythms.
What to Avoid
Whether you’re writing harmony or counterpoint, there are certain combinations
of notes that you want to avoid. In most cases, you’ll avoid these combinations
because they don’t sound right; still, it’s good to know the conventions in advance
so you can keep from making rookie mistakes.
Avoid Extended Parallel Movement
Parallel movement is when two or more voices move in the same direction by
the exact same interval. Parallel movement is frowned upon in some types of
music—but perfectly acceptable in others. For example, in classical music you
want to avoid parallel movement in perfect fourths or fifths and octaves; in popular
music, however, parallel fourths and fifths are permitted and even somewhat
common—even if they do sound a little boring if carried on for too long.
In any case, if you try hard, you can generally come up with something that
sounds more interesting than extended parallel voices. Go ahead and use a little
parallel movement if you have to, but then vary things whenever possible to
keep from getting boring.
Avoid Big Leaps
Whether you’re writing harmony or counterpoint, you should follow one of the
key conventions used when creating melodies—keep the intervals between
notes as small as possible. Avoid big leaps between notes; they not only sound
disconnected, they’re also hard to play and sing. As with standard voice-leading,
any interval larger than a fourth or fifth is probably too big a leap.