1) Depending on the length of the piece, give yourself several WEEKS to get memorized. If you don't have weeks then you'll have to cram. Better memorization comes over a length of time.
2) Work on it EVERYDAY, or almost everyday. It doesn't need to be long work sessions, just a bit everyday.
3) Write or type up and print out, then highlight the speech (monologue, presentation, etc.) in different colors. First, writing it out helps with the brain catch on. Second, highlighting sections of your piece in different colors gives you a visual indication of the words--again, helping your brain to catch on.
4) Record it. Use whatever portable device you have handy and record your presentation. Now play it back as you are in the midst of physical labor that doesn't require you to think much--cooking, yard work, cutting firewood, walking, feeding the birds, driving (but do stop for the red lights).
5) Take it sentence by sentence. Read sentence one. Now read sentence one and two. Now read sentence one, two and three...keep going. You can also take it paragraph by paragraph.
6) Give yourself cues. A PowerPoint already has your bullet points of ideas or pictures to let you know what's next. If you have a speech you can make pictures for, do it. If you're won't have pictures for what you're doing, draw reminder doodles on the sides of your paper to use as you memorize.
7) Come up with gestures to go along with your piece. Always do the same gestures at the same places during your monologue. It's just another cue to get your brain to grasp the words.
8) Get a helper. Let them hold your script and correct you as you flub. At some point, you've got to put your paper down or that will turn into a physical cue for your speech. That's why so many people can recite something they've memorized as long as they can hold the paper in their hand. So put down the paper often, too.
9) Repeat. Any of these steps can be used over again as you get your speech memorized. Use them constantly and you will get it!
Take heart. Memorizing is possible. And you might find other ways to memorize than what's on this list.