Say nothing negative

When I observe presenters, I see a lot of this crap:

• "I didn't really prepare, and I don't know all this stuff, but here goes."
• "My boss was supposed to give this presentation today, but she's out. I'll tell you about this, but I'm not as good as her." 
• "I'm not the expert on [Topic], but I'll go through this anyway."

Why add a negative qualifier up top? What good does it do the speaker to give the audience an immediate, negative perception? 

If you set an expectation for crap, the audience will expect crap.

Never begin with "I'm not the expert." Really? You're not? Then why am I wasting time here?

You don't have to know every single thing -- but you do have to be professional. Kick off your presentation with a good start, and say nothing negative. There's no reason to put a bad view in your listeners' minds. Show us how good you are -- and what you do know.

Better openers are:

• "I'm happy to talk to you today about [Topic]."
• "Susie is unexpectedly out today, but I'm stepping in. If you have any questions, I'll do my best to answer them at the end." 
• "I've been working on [topic] for six months, and I'm here to present [whatever it is]."

That's all you need to say. No qualifiers about what you do or don't know. If you get a curveball question, say, "Let me check on that. I'll get back to you," and then do so.