When I first started doing public speaking with Toastmasters, I did not know any other way of memorizing speeches other than writing it out and memorizing the speeches word for word. This worked for me for a while but I actually remember one speech where I lost my way completely and just for the life of me could not recover. Since that point on, I decided that memorizing speeches word for word was certainly not the best way to go because it did not allow for me to fail gracefully in the event that I could not remember my speech. I switched over to memorizing concepts instead and only memorizing quotes or key statements that I needed to get the wording right and while I can't say that I haven't lost my way from time to time, I can say that memorizing concepts has allowed me to fail more gracefully.
How can speakers recover if they forget what they need to say next? Here are three strategies that I have both seen and used when I'm up in front of the audience:
Repeat what you talked about at a high level
Let's just say that you are presenting a speech where you have three main points. You finish talking about point 2 but when you get to point 3, you completely forget what it is. Rather than trying to stumble your way through point 3, you go back and re-iterate point 1 that you talked about and then point 2. It's a way of buying some time to think about what point 3 was AND it might trigger your memory when talking about point 1 and point 2 (similar to how you would have practiced your speech).
Go to your notes
Same situation - you forget your way and forget what point 3 is but you have your notes with you (either at the lectern or somewhere close by). Here is what you can say:
"I am going to stop and take a look at the exact words because I want to get this next part just right"
It gives you a chance to look at your notes, remember your key points and then continue on with your speech.
Memorize your concepts backwards
It sounds weird but it makes a lot of sense - when you remember things in a certain order (number 1, 2 and 3) then number 1 will be fresh in your mind, number 2 a little less and number 3 a little less than 1 and 2. In a weird way, it almost makes sense to spend the time and energy to memorize number 3 and rely on your short term memory for number 1 and 2 - by the time you do your speech, number 1 and 2 will be at the top of your mind and you do not need to spend the time and energy to memorize point number 3 (so you might as well focus on it when you practice).
I hope you don't lose your way during presentations - the skill of memorizing speeches these days is few and far between given that everyone has notes with them but nonetheless, it is still an impressive skill to have and can make your presentation better because it's one less thing you have to worry about.