Maintaining your own harps can give you longer lasting harps that play and sound better.  Just as guitar players have to change their strings and constantly tune, or sax players have to work with their mouthpiece, harmonica players should be able to set up their instruments to sound good and play well.  Maintenance of a harp most often centers around the reeds, as shown in this picture.  The rivets attach the reeds to the reed plates, and the reeds vibrate through slots in the reed plates to generate the sound.  The action of the reeds depends on the gap between the reed and its slot in the reed plate.

Tuning

Tuning reeds is done by either removing (normally) or adding (infrequently) metal from/to the reeds.  Here's how it works

• To raise the pitch of a reed, remove metal from near the reed tip (see above picture).  This lightens the tip of the reed, allowing it to vibrate faster, which raises the pitch.
• To lower the pitch of a reed, remove metal from near the reed base (see above picture).  This weakens the reed and makes its tip heavier relative to its base, which slows the vibration and lowers the pitch.
• Alternately, to lower the pitch of a reed add lead-free solder to the tip of the reed to increase the weight at the tip and cause the reed to vibrate more slowly.  It is also possible to increase the weight near the base of the reed to raise the pitch.