Tips and Tricks
Because harpists only use 4 fingers it's important to make sure chords are manageable. Harpists are used to having chords of an octave or a 10th but if you want the two notes in between the chords become more difficult. Typically chords in the left hand are the octave with the fifth (e.g. C, G, C). Lots of chords in a bar can be difficult for a harpist to play, and less effective as the string has less time to ring, so one chord per bar is normally more effective.
Chromatic progressions on the harp cause problems, because you get lots of noises from the harpists feet. The harpist will also have to damp chords before playing the next one, otherwise the notes of the chords will change when the pedal is moved. This means that the natural resonance of the harp is lost, so it's a good idea to restrict chromatic movment on the harp as much as possible.
When the harp is in its lowest keys (i.e. flat keys where fewest pedals are depressed) it resonates better as there are no discs or mechanism stretching the string. This means that writing in Db major (Bb, Eb, Ab, Db, Gb) will sound much more resonant than B major (F#, C#, G#, D#, A#). Writing in the flat keys will also give more options for modultation as it's difficult for a harpist to modulate easily once they're fixed in a 'sharp' key like B major.
Repeated notes on the harp are not a great idea, and repeated chords are even worse - because to play a note you have to stop the strings ringing it's difficult to play repeated notes as Miriam demonstrates.