I know this doesn't add too much, because the basics are all here, but from my experiance, unless you are using thin orchestration to highlight the harp, a harp's parts will primarily focus on the attack of the note--particularly in the treble. The bass notes will reverberate, and the harpist can silence them by putting his/her hands on the strings at the end of the note value. However, the higher the pitch, the shorter the string and thereby the shorter the audible ringing will work.
The attacks of the notes is really what comes across to the listener most in a louder moment. I had a piece read recently with an accompaniment-like harp part during a fair volume level moment with several instruments...the attacks come across, but its not much more than a nuance.
Now to get some more info from the bible...i mean Berlioz... :D
Keep your lower harp parts open, especially below G2, though I'd contemplate stopping at C3. Low range chords on harp are very muddy.
Berlioz brings up another interesting point I had forgotten: (harp is not my favorite instrument to write for) Keep the hands at least a sixth, preferably an octave apart. Arpeggios in thirds will therefore, due to the overlap, have no sonority due to the rearticulation of the strings.
Fairly fast octaves should be written for two hands, rather than one, solely because the thumb can't be expected to move as fast as the other fingers in succession.
If you would like a note to be re-articulated quickly, use the pedals to your advantage to allow vibration. So rather than just a B flat marked for thumb and index finger, use A sharp and B flat. Or, if you would like a repeated chord, use two harp parts and have them switch voices every other note.
Berlioz offers a lot of information. If you are really intrested in the harp, drop me an IM sometime or a message, and I'll try to scan some pages for you. It's worth the read! :mellow: