Listen!
Have you ever heard a song that was so catchy that you couldn't forget it? Sentences can be like that too. It's a lot easier to remember the sound of a sentence than to remember each individual word in order.
Close your eyes and imagine.
You remember things better when you have feelings about them. Don't think of English sentences as information. Really imagine what you might think and feel as you say the words.
Memorize backwards.
Instead of reading the sentence through from the beginning, try picking a few words at the end, repeating them, and then adding more words, bit by bit. I describe this technique in detail in this article: http://www.phrasemix.com/blog/a-cool-trick-for-memorizing-sentences
Say it out loud.
Remember, you're not just teaching your brain to speak English. You're teaching your mouth too. When you practice sentences, repeat them out loud. If you're in public and you can't speak out loud, then mouth the words silently.
Repeat, many times.
You can't learn a sentence by repeating it 5 or 6 times. You will probably need to repeat it 20 or more times to get it. You'll also need to review it again in a day or two.
Now you know how to learn sentences. The next question is, which sentences should you learn? Tomorrow, I'll teach you how to pick the best sentences.