Making yourself heard at work

I announced to the lovely people over at Maya's Mom that I was starting an advice blog and someone imediately responded with a question. It's an auspicious start! Before I answer though, let me reiterate again, I'm no professional, I just see what I see, and say what I think. (And I really feel like a fraud... but the friends and the husband, they keep presuring me to do this...)
Here goes nothing!

"Many times now this has happened with a number of different people at work. I suggest an idea and it is overlooked, not heard, or disregarded. Then a while later someone else says it and everyone says "great idea!" What should I do?"

Clearly your ideas are not the issue. If people like them when someone else brings them up, I have to deduct that the issue has more to do with your delivery than the content.
Is it possible that you have gotten to the point where you expect people to ignore what you say so you are not saying them convincingly enough?
You need to have faith in yourself. Your ideas are good and you need to believe that when you present them. Wait until you have everyone's attention. Make sure everyone is looking at you. Use strong language, no "I think" or "Maybe" statements. If you project doubt people will pounce on it. Your body language is important too; sit up straight, make eye contact, don't speak too fast. The more confidence you exude the more people will pay attention. And if you don't believe a word of it? Fake it. Picture the most self confident person in your office and pretend to be her. The more you pretend to be a self confident, self assured person, the more it will become true.
And if it doesn't work the first few times? If people still ignore you and claim your ideas as their own? Then speak up! Use sarcasm even if it helps "What a great idea, I'm glad I came up with it first!" You'll want to puke the first time you do it, but you have to be the one to defend and stand up for yourself. After a while people will be used to it and start to do the same. If you let people walk all over you, they will. When you respect yourself and your ideas people eventually follow suit.
So stand up for your ideas! Be proud of all you have to give.