OK, I’ve given you lots of ways to rehearse.  You won’t do all of them, unless you really want to be good and you have the time, but you should at least rehearse several times before a big presentation.  The alternative, winging it, is never as good as you think it is.  And your audience won’t tell you the truth. 

Unfortunately, what happens is that the speaker who wings it gets pumped full of adrenaline, comes charging off the stage and asks the first person he sees, ‘how was it,’ with a big smile on his face.  Only a churl would reply with, ‘well, it was disorganized, there were lots of minor screw-ups, and you kept making the same points over and over again’.  Most people say, ‘it was great!’ and the speaker think to himself, ‘That’s all right then; next time I’ll do it the same way.  Obviously I’m too cool to rehearse’. 

In that way bad public speaking is perpetuated. 

Now, don’t get me wrong.  There are times when off the cuff remarks are fine — indeed, preferred.  It depends on the formality of the occasion.  The less formal, the more off-the-cuff is OK. 

I’ve often heard it said that, for most men, the first time they have to give a speech is when they’re called upon to give the toast as the best man at a friend’s wedding.  I don’t know if that’s factually true or not, but it certainly calls up a host of memories of bad best man speeches.  That’s a formal occasion, last time I checked, and deserves a little preparation.

Informal occasions where you might wing it are regular meetings where you don’t have a prominent spot on the agenda.  That sort of thing.  Where the stakes are low.  Where it would look odd to have prepared remarks.   

Ultimately, the problem with winging it is that it shows that you don’t think the occasion is very important.  That sends out a message which can easily be taken the wrong way by people who do think it’s important.  You have to decide whether it’s worth it to anger those folks for the sake of avoiding a few rehearsals.