It should be obvious that you have to rehearse your presentation before you give it. Although, given the fact that I'm often seeing speaker overrun their time slot, I have to wonder if it's really something many speakers do. I get the impression that not rehearsing (at all or earnestly) is the number two reason for going over time (number one would be trying to cram too much content into the presentation).
Assuming you do rehearse: Is there an upper limit to rehearsing? I.e. can you rehearse too much?
What to rehearse
Obviously, you should be rehearsing your entire talk a few times, so that you know how long it will take and so that you get a feeling for its flow. You can also define milestones this way: "20 minutes in, I should be at this point" or "When I get to this slide, I should have 15 minutes left". These milestones will help you adjust the talk in case you're about to run out of time, i.e. so that you can skip things to make up for unplanned events, such as discussions that took longer than expected.
The two parts of your presentation that you should be rehearsing a few extra times are the beginning and the end. You will have the audience's full attention at the beginning, so you want to make best use of that phase to start with an important point that you want your audience to remember. This will work better when you're more confident and therefore more convincing. It will also help your own confidence and reduce your nervousness; I find that a smooth opening helps me get into the flow of the presentation more easily and any nervousness will evaporate quickly.
The end of the presentation is just as important since the audience will remember the end very well, too. So again this is something that you should have down pat. A strong end will also make it clear to the audience that the talk is really over and that they can applaud now.
The bits in the middle
So what about the part between the beginning and the end? There will usually be a couple of important points and transitions that you want to make sure to get right for maximum effect.
For the rest, I'd say that as long as you know what you're going to be talking about, you don't have to rehearse them a lot. I have a few presentations that I have given several times now and I find that I always phrase things a little differently each time. And that's fine! As long as you don't forget anything of importance, it doesn't matter. On the contrary, it will come across as more natural, even if you make a few mistakes along the line. If you rehearse a lot, you risk ending up reciting practised lines which can come across as less natural, too "smooth“, and therefore less convincing.
It's important to rehearse, there's no doubt about that. It is especially important to rehearse the beginning, end, and important points in the middle. For the rest, assuming you did rehearse the entire presentation a few times, it's better not to overdo it.
(Image Credit: at the ballet dress rehearsal : san francisco opera house (2012) by torbakhopper, from Flickr, CC-BY)
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