You have very small O ring type apparatuses under your air bag which control your horn. They become worn with age and use.
Here is what you will need to do, stop once you remove your air bag:
http://www.whiteson.org/boxster/mods/obc/obc.stalk.pdf
Read this thread:
http://www.renntech.org/forums/index.php?showtopic=1728
Scott