Speaking in front of people can be a daunting task. You have to stand alone in front of a group of people, and whilst everyone may be encouraging, you can still feel the nerves eating away at you from the inside out. If you do have trouble speaking in front of people, then you may like to read a very early post of mine: Dealing with Public Speaking.
For other people, speaking in front of others is as natural as breathing and they excel in the spotlight. Having a speech already prepared is a great aid, having a speech memorised though gives the appearance of professionalism and assurance, helping to convince the audience of what you're trying to convey.
However, even the people who are gifted at speaking in front of others can have a hard time memorising a speech, and memorising it to a point where you know you won't forget it whilst in the middle of delivering it.
1. Don't leave it until the night before
You will find posts that tell you that you can memorise a speech the night before, and for smaller speeches this may be true, however for longer speeches, you're not going to be able to get to a level where you can deliver it with the justice it deserves - unless you have an eidetic memory or something. :)
A lot of these websites that exclaim this feat will actually have some good and valid tips about how to memorise a speech, but for a lot of us - not in one night. If we hypothetically say though that we can memorise a whole speech in a night and then are able to deliver it on the day word-for-word; if we can do that in one night, imagine how much better we could get that speech by tackling it in plenty of time before - hopefully leaving everybody else speechless. :)
2. Read through it aloud
Make a point of reading through your speech aloud at a steady pace. This will allow you to get a handle on how to pronounce it and tweak things that will help you to sound less stiff and awkward. Having a better grasp on your speech and reading through it aloud will make the process of memorising all the easier easier.
3. Repetition
The only way you're going to be able to get the speech into your head and for it to stay there is by using repetition. Each time you read it you are re-familiarising yourself with it. Try to read through it aloud 5 or more times a day, every day. You could do one in the morning, one at midday, one in the afternoon, one in early evening, and one in the later evening. Before you know it, you'll be reading parts of it without needing the written version, and eventually won't need it all together.
4. Memorise sections
If you have a small speech, then you will probably will be able to start memorising the whole thing. However, if you're faced with a ten-page essay - you will need to break it down to more manageable chunks.
When you have a section that you want to memorise, you will need to read it out loud a number of times through the day (basically the same as point #3). However, what's different from point#3 is that after every time you've read the section, put it to one side and try to remember what you've just read. Try to get as far as you can and don't look. When you can't go any further (even if it's only two words in), pick up your section a re-read through it out loud again.
Leave it after you've read through it a second time. Later on when you come to re-read it - repeat the whole process. After a day or two, you'll be seeing some results. This is why I said that it would be a good idea to start preparing for your speech ASAP.
5. Mnemonics
A great thing you can do to memorise not just a speech but anything in general, is to use the technique of mnemonics. Mnemonics sees you link something to the thing you're trying to memorise to make it easier to memorise. For example; pictures are often a popular mnemonic as the visual aid jogs the persons memory. You could use visual aid prompts for each section of your speech. Every time you are trying to memorise a section, have that picture in close view to look at, or have pictures in a presentation when you're delivering the speech to help you. You'll be surprised how well mnemonics work if done properly.
Some other types of mnemonics involve using: sounds, pictures, songs, prices, acronyms, stories, numbers, phrases, the method of loci, and much more.
6. Write it out
This may sound like nonsense but writing your whole essay out will help you when trying to memorise, as long as you are concentrating on it whilst you're writing it out. It may take a while if you have a long speech, but writing it out will help your mind get to grips with it better and will give you a better knowledge of your speech. Some people's minds memorise things better when they write something down, so it's beneficial for them, but it's also beneficial for others also, it's just that bit more beneficial for them. :)
7. Memorise the structure
You don't have to memorise your speech word-for-word, only if you want to. Memorising the structure and layout of your speech is a far easier and quicker thing to do. Then after you've done this, as long as you have a quite in-depth knowledge of your speech and what's included in each part, then you can talk about it when you're in the moment. Of course it would be wise to rehearse it so you become at least familiar with this concept. If you do use this technique then you wont be as rigid when delivering it and it should hopefully be more spontaneous.
8. Memory cards
Get yourself some cards that are large enough to write sections of your speech in bold clear writing but are yet small enough to fit inside a trouser pocket. At various points throughout the day take out these cards and read through them, trying to memorise them if possible. You can either memorise them word-for-word or just the general outline - whichever is better for you.