It only makes sense that you need to figure out what you are going to say, and also practice how you are going to say it, right? As an actor, I am used to doing a lot of practicing (we call it rehearsal). And most actors will tell you they never have sufficient time for rehearsal. Our process is a lot more in-depth than what speakers go through, of course. Speakers are delivering speeches that they have likely written, in their own voices. Actors use the playwright's words and speak in the character's voice. But in the beginning, the actor's relationship to his script is very similar to the speaker's relationship to her text. Know what you are saying, what your intention is, be aware of subtext is (i.e message beneath your words/between the lines), be sure you can pronounce all the words/names, etc. Then practice enough so you don't have to read or stay glued to your text. And when you have internalized the message, you are ready to increase the dynamism of delivery with more energy, more vocal variety, better pacing. The more you know your text, the more expressively you can convey your meaning. And the more expressively you do that, the more vibrant you will be. Fresh, never "stale." So you can see why it is hard for me not to laugh when someone who needs to do a speech tells me he is afraid of being over-prepared and sounding "canned."
The fact is there is no such thing as being too prepared.
Every time you speak in public, in a formal speech setting or around the boardroom table, you have an opportunity to prove your expertise, underscore your credibility, convey your dynamic leadership. Why would anyone leave that up to chance? "winging it," "speaking off the cuff" and other techniques that rely on the inspiration of the moment may work for you some of the time (I have observed, unscientifically, that this figure hovers around 25%). Why chance it the rest of the time?
Think about it: the last time someone really knew her stuff, did you think she was "canned?" Or prepared to perfection?