Practice makes perfect
The only way to become good at any task is to practice it. You can have the best speech or great talking points on an index card but if you have not actually practiced verbalizing the words, the chances of it sounding the way you want it to and being effective is about 50/50.
Think about how much athletes practice. They spent hours each day doing the same thing over and over. In fact, Geoff Colvin, in his book, Talent is Overrated, suggested that it takes a minimum of 10,000 hours of doing something to become proficient at it.
So, if you want to be good at any task always take time to practice. If you do this, the chances are that you will knock it out of the box. Otherwise you could appear stiff like Clint Eastwood and his ad lib performance at the Republican convention instead of President Bill Clinton’s rehearsed one at the Democratic convention.