From Jon Jory's book, "Tips: Ideas for Actors"
"My father (Victor Jory) acted in 137 films and hundreds of plays. He was a slow study and I spent a good part of my childhood cueing him. How did he doggedly learn lines? He underlined the last five or six words of the speech that gave him the cue. He took a large index card which he wold obscure his own line. He would read the cue out loud (so the sound of it would get in his head) and then reply with his speech in a whispered monotone (so he wouldn't set a reading). He would go on until he forgot a phrase or line. Having forgotten, he would study the offending line, say it several times and then go back to the top of the page. He never, ever, cut himself any slack; he always went back to the top of the page. When he knew an entire page, he would go back to the beginning of the scene or act and cue himself up to the same point, where he would resume memorizing. Arduous? Indeed. But until his early 80's, I never heard him drop a single line. Once a scene was blocked, he never carried a script. 'Unprofessional,' he'd say."
I love hearing how the old school actors did it. Now we have cool sites and phone apps that can help actors study lines. Here's a good cue card tool that you can use: www.studydroid.com It is also an app that you can download on your smart phone so you can study your lines anywhere and anytime.