Over the years I’ve watched loads of presentations all over the place from students, professionals and even once, by mistake, from me on Channel 9. And one thing that has struck me is that there is one thing you must never, ever, do in front of an audience. And that is apologise in advance. If you say at the start of your talk “I’m sorry, but because I’ve not had time to prepare/got drunk last night/just ended an unhappy love affair (delete where applicable) this is is not going to be as good as it might be” then your audience is instantly expecting you to fail, and they will set their expectations appropriately.
Now this goes against a certain aspect of “Britishness” which is that we from the UK should at all times to be slightly self-effacing and modest. If a Brit tells you that something they have done is “quite good” then be prepared for something truly amazing. But you would never anyone from the ‘states saying that something is quite good. In America things start at “awesome” and then go up from there.
I’m not saying that you should attempt to deceive your audience, or that you shouldn’t apologise if something stops you from doing a good job during the presentation. What I’m saying is that you should not set the wrong expectations at the start. I’ve seen some superb talks from people who have come off the stage and told me how badly they thought it went. I’ve also done what I thought were barnstorming presentations to get a decidedly ho-hum reaction from the audience.
If you are not sure about something, say so during the presentation not at the start. If you put a downer on things right at the beginning you are not actually doing yourself any kind of favour.