4 Answers

Tips here>>http://www.ace-your-audition.com/how-to-memorize.html

The few acting opportunities I've had, to learn my lines (which I had to do, as we didn't have cue cards), was by practicing over and over and over.  During practice, I didn't just learn the lines as words, but the expression that went with them.  It's like learning anything else (piano music, facts for a test)....practice. 

I don't think they memorize much at all, they do it in 'takes' A few lines then stop, memorize a couple more lines do another take until the whole movie is filmed to the directors perfection and they yell 'Print it!' .

I think you have to sing about many lines as your brain can take for the first week then you keep adding for the weeks to come