First write down whatever you have to memorize in the speech you are planning to give. After you are done with writing it down, type it down so that you are also revising it in a way. Typing is necessary because on a computer there are not problems of handwriting and you can clearly see what you want to memorize.
Once you have written down everything then read it and record it on a tape, or even on your phone. This will really help you revise whatever you are trying to memorize. First of all, you will be saying things out loud that will help you recall it when you are giving the actual speech. Moreover, you can later listen to the speech and it will really help you remember things.
Listen to the recording that you have done before. It will really help you remember each and everything. Moreover, when you are listening to the speech, you will know where to put power in your voice and will know how to say things differently, in order to make an impact on the audience.
There are definitely different segments to your speech. It could be the number of points you are trying to make, or the different scenarios that you are trying to explain. Therefore, you have to memorize each and every segment one by one. Do not move to the next segment before you are done with the first one. It is absolutely necessary to remember each and every segment and even if you do not remember all the things which are a part of the segment, it is okay.
Revision is absolutely necessary in this case. Keep listening to your speech again and again. Try to speak alongside it and when you are doing it, you will learn simultaneously.