I think a lot of people giggle at inapropriate times.
I havnt really got any real advice for you becuase I do the same thing. but one thing which might help is to concentrate fully on what is happening, and try to get in the mindset that you ARE being punished and try to concentrate on not being punished again.
Another thing is to sit down with your Master/Dom and explain what you are feeling and why youthink you giggle and maybe try the punishment in different ways or on different levels and see if any of them stem the giggling?
xx