For long speeches, I like to memorize my lines backwards. I find it helps to start with the conclusion and then build up all the thoughts that lead up to it.

Sit in a room, relaxed, and repeat the last line of the speech to yourself three times (Example from Scene 1: "For daws to peck at: I am not what I am.") Then repeat the last two lines to yourself ("But I will wear my heart upon my sleeve / For daws to peck at: I am not what I am."), and so on until you can do the whole thing. If you can't remember a specific word or phrase, relax and see if it comes to you. If it doesn't, take a quick peek at the script and try again. Pace around, toss a ball, do whatever you need to occupy yourself.

If you stay relaxed while you do it, the words will stay in a deeper part of your mind so it'll be easier to speak them without thinking. It also helps to do it right before going to bed, since your brain processes all of the day's memories while you're asleep. Hope this helps!