Just as there are ways to grab an audience's attention, there are traps even experienced speakers fall into that work swiftly to halt whatever momentum you're building:
Don't use the opening to restate the title of the speech or to reiterate information. You need every moment to create interest and suspense; don't go over what is already known. Don't start by saying, "I am going to talk to you today about...safety."
Don't open your speech with an apology. You may think it makes you sound friendly and not pompous, but apologies set up your audience to listen for your weaknesses.
Don't greet the "important" people in the audience. Forget saying "Thank you, Mr. Chairman, Mayor Jones, Senator Smith...." The only time you would use such a formal opening is as a political candidate speaking before a very distinguished audience. If you want to bring attention to certain people in the audience, use their names in the context of your speech.
Don't explain your presence. Don't offer explanations about why you think the chairman asked you to address the group. Remember that you are there for a good reason; you know it, and the audience knows it. Also remember the maxim covering explanations: "Your friends don't need it, and your enemies won't believe you anyway."
Don't say how difficult it was to choose the subject. As far as the audience is concerned, your topic should be so vital that you never doubted its importance, and you should communicate that vital nature.