I've done a lot of public speaking since high school, I still get nervous. The amount varies depending on the size of the crowd. Before speaking, take a few minutes and breath. Inhale through your nose and exhale out of your mouth. It's scientifically proven that doing this, will actually slow down the adrenaline pumping through your system.
Do some loosening up exercises. Seriously, stretch everything. Your shoulders, arms, legs, back, and your jaw. This will also help you relax and the best speaking always happens when everything is loose and relaxed.
If you can have note cards, then do so. Make them small and don't write your speech on them, but rather important points. These can be a great aid to help keep you on track if you feel like you might forget something.
Practice, practice, practice. Then practice some more. Get comfortable with your speech, the sounds of your words. They should flow from you like a smooth and calm river. If your speech is memorized, close your eyes and imagine the room you'll be speaking in. Imagine 100 faces, and make your speech. This helps set your mind to the moment.
Eye contact is key. As another poster said, you should speak to the crowd as though it were just one person. I like to find key faces around the room to focus on. Usually 3-4 and speak to them at different intervals to make that appearance of speaking to the crowd. I know someone said this above, i'm just too lazy to copy the post...lol. It's all true though.
The big thing that will make you better at speaking, is to continue doing it over and over again.