Many (Most?) of us work a day job - probably in an office setting. Working as part of a team in an office can be great, but it also has it's challenges. Being heard, even if you have great ideas, is one of those challenges. Your instincts may tell you to speak louder, and more often. After all, if you're always outspoken, always loud, people have to listen to you, right?
Not so fast, buckaroo.
In fact, the complete opposite is true. Think for a moment - you probably have that guy in the office who just won't shut up. He speaks up in every meeting, trying to take it over. He's constantly giving his opinion on things, even if they don't really concern him. Regardless of the subject matter, Mr. Chatty has something to say.
Do you listen to this guy?
I don't. And I'd wager to say that you don't either. This guy is talking all the time, and that dilutes his effectiveness drastically. You aren't likely to take his thoughts seriously.
Do you want to be heard? Shut up.
Don't voice your opinion at every opportunity. Keep your thoughts to yourself; let others take the stage. That is, unless you feel very strongly about the matter at hand. By keeping quiet in most situations, when you do finally speak up, everyone is very likely to stop and listen - really listen.
Choose your timing carefuly, and choose your words carefully. Make sure every time you speak, you have a clear purpose. Make sure your words are important, and that they will make a difference. Do that, and your coworkers will truly listen to you. Your words will matter - and all it takes it learning to keep quiet.