If you play a harmonica regularly, it will need cleaning after a while to keep the holes and the airways under the reeds clear and in good working order. The best way is to soak the harmonica in a bowl of warm water.
Turn the harmonica over and shake it several times while fully submerged to remove any air trapped inside it. Leave the harmonica soaking for about an hour, and then alternately shake it and dip it in the water several more times. Also, use a small implement to clear out any unwanted matter if necessary.
The next step is to run the harmonica under a cold tap several times, shake it and then tap it on a semi hard surface to clear the water out. Finally you should try exhaling and inhaling through all of the holes until you are confident they all sound properly. If not, shake and tap the harmonica some more until all of the water comes out.
Harmonicas with a plastic body respond better to cleaning than the wooden bodied type, as the wood will often swell and can cut your mouth when sliding between notes.