You don't need to.
Learn to cry.
You don't need to learn how to scream, to rage, to hurt, to hurt someone. You're human. You know how to do that. Just like you know how to laugh, to shout, to celebrate life. You don't need to practice being human. You also don't need to "become" the character. If the piece is well written, you already are the character. He or she is human. You share levels of experiences and understanding. Plays and movies are usually about profound, exceptional and dramatic occurrences. Sadly or joyously, you share these with the character. At the very least, you have the capability to understand what they are going through. You're human. The story, the journey of the character will prepare you "to cry". The character doesn't start it's life thinking "I cry in two weeks". The character is living and when great sorrow comes to her life... she cries. Or tries very hard not to. The most wonderful and tragic moments of life are usually punctuated by people trying not to cry.
Trust the writers. They're human and they have written something to celebrate life. Even if sometimes it's the darkest parts of life.
To be a performer, you need to learn tell a story. To be engaging and entertaining. You prepare by knowing your lines, your blocking, the story, the world of your character so well that you are "alive". The writer will give you reason to cry "two weeks later".
And you will try not to.
Because you're human.