Sometimes, NOT SHUTTING UP is the right thing to do.
However, that said, I'd try biting my lip a bit and saying whatever it is that you might WANT TO SAY to yourself, instead. That way, you are thinking it and kinda "saying" it...but the actual words don't come out.
Try LISTENING more...not just HEARING THE WORDS...but hearing the thought or concept itself. People who can't shut up are usually poor listeners. They are so busy forming their next thought that they are not really listening to the other person. Improve your listening skills and you might find it easier to shut up...because you will be too busy listening to be able to talk.
hehe - let me know when you find out - i have the exact same problem :)
Why should you shut up? Is it because of what other people might think? Try not to care about that, it is very liberating.
Maybe you should get one of these:
Once you learn how to shut up, you can have the best of both worlds: win friends, then subtley influence them. =)
Not shutting up can be a good thing. It is beneficial to hold strong to your beliefs. However, it is best to be tolerant of everyone else's as well. Appear intolerant, and you will not have many friends. If you allow others to be outspoken, they'll show you the same courtesy.
Tact, however, is very useful. Know WHEN to shut up and when to argue. That's something you should practice more than shutting up in general.
You should get one of those devices that delivers a mild electrical shock. Just shock yourself whenever you open your mouth without thinking and you'll soon learn to stop :p
Though personally, I'm quite fond of very opinionated people. If no one spoke their mind the world would be a very boring place indeed.