Someone sent me an email today asking whether word-for-word memorization was the best method for preparing for a big speech.
Generally speaking, I never encourage anyone to memorize a speech. When you memorize something word-for-word the delivery becomes artificial. Also, if you suddenly 'blank out' it's very hard to pick up the pieces.
That said, sometimes memorizing small parts of your speech like your introduction and conclusion can help. Your introduction is one of the most important parts of your talk. If you fumble on your introduction you can lose some of the audience at the very start. Even worse, your confidence can falter and that can have a negative effect on the rest of your speech.
Another good reason for memorizing your introduction is because it can help with a fear of speaking. Have you ever noticed that you're most nervous at the beginning of your talk? After a few minutes when you see that everything is going well and your confidence builds, your nervousness starts slipping away. A great introduction isn't only great for your audience - it's also great for you!
Your conclusion is equally important. You want to leave the audience feeling good about your subject and, more importantly, you. You don't necessarily need to memorize your ending, but have a really good idea about how you want to end. You want to make a great impact! If you feel more comfortable memorizing, that's fine, but make sure that your delivery is spontaneous and authentic.
Other than these two parts, I suggest that you memorize your main points in outline form. You should definitely know what you want to say and where you're going, but trust yourself to find the right words in the moment. You will come across as being more honest and authentic. Let yourself speak from your heart and your audience will definitely respond to your message.