People often ask me about rehearsal in the hopes that I’ll say, "Sure, don’t worry.  Don’t rehearse too much or you’ll get stale.  Better to keep it fresh.  Wing it."

I never do say that or anything like it.  More rehearsal is better, with the following caveats. 

Don’t rehearse more than once on the day of the presentation.  I can imagine exceptions to this rule, but not many and not often.  By the day of, most people are in adrenaline mode, and rehearsal is not very helpful.  Rehearse once, especially in the actual venue, just to get familiar with things and have the performance fresh in your mind.  But obsessive rehearsal at this point simply won’t do much good.  You have to have done the work already.  It’s too late.   

Don’t rehearse the wrong speech or presentation.  This may sound odd, but you’d be surprised how many times people don’t have a speech set until the last minute, so if they do rehearse, it’s the ‘wrong’ speech — because it’s not the one they’re ultimately going to give.  Get the speech set, weeks before the date, and rehearse that one.  Many people get nervous as the awful date approaches, and they start to doubt themselves and the message.  So they tinker with the speech, almost always making it worse.  Don’t fall into that trap. 

Don’t rehearse too often in your mind.  Half the reason for rehearsal is that speaking, like, say, acting, is a physical art.  You rehearse so that your body can learn the speech, not just your mind.  Too many people say, "I don’t need to rehearse, I ran over it in my mind."  Therein lies potential disaster.  You need to discover physically what it feels like to give the whole speech, to say a particular line out loud, to make the transitions from one section to another.  None of those things can be imagined as effectively as they can be rehearsed. 

How much should you rehearse?  A lot.  Stage actors often get 6 weeks, 5 or 6 days per week, 8 – 10 hours per day, to rehearse.  That’s how you end up looking natural, assured, and authoritative.  Not by winging it.