The key to memorizing a long passage such as a speech is creating memory associations rather than simply memorizing each word in sequence. Focus initially on learning the outline of the speech and not individual words. After comprehending the overall scope of the speech and then linking it to the supporting concepts, memorization is much simpler.
Before attempting to memorize a speech, read it in its entirety and be sure you understand what each part means, as this comprehension is necessary before meaningful associations can be made. Once you understand the speech's message, read it aloud, making note of any parts that cause you to stumble. For each of these points, associate an image with the concept. If convenient, draw a small picture of the image to make a personal connection to it.
Continue to recite the speech, relying increasingly on the images as reference points instead of the text. Gradually reduce your reliance on the images by visualizing them rather than looking at them until the speech is memorized. If this proves challenging, try starting at the end of the speech and work backwards, adding memorized phrases and sentences one at a time.
This technique is especially effective at creating a more natural performance. Memorizing the outline rather than just specific words allows the speaker to improvise when appropriate and helps reduce the artificial-sounding delivery common to untrained speakers.