How do those actors and actresses memorize hundreds of lines? Memorizing lines takes practice and constant repetition. However, there are a few ways to make the memorization process run smoothly and quickly.

Read Out Loud (And Repeat, Repeat, Repeat):
For most performers, there is no short cut to memorizing lines. To learn lines, an actor must recite the play out loud, over and over again. Most rehearsals encourage this by running through the lines or having a read through.
Record Your Lines:
Because there is often not enough rehearsal time, many performers find
ways to listen to the plays dialogue during everyday activities. Turn your work-outs, chores, and recreational activities into a read through with the help of your headphones or your i-pod. Aside from constant rehearsals, this method seems to be the most popular way to memorize lines. Use a tape recorder or better yet an MP3 player to capture the lines from each relevant scene. Others opt for recording the lines of fellow cast members, but they leave a blank space so that they can insert their dialogue while listening to the recording.

Think Positively and Dont Panic!
Dont let the butterflies in your stomach torment you too much. Most people  experience stage fright minutes, hours, even weeks before opening night. While a certain amount of nervousness can get the adrenaline going, too much anxiety over lines might hinder an actors performance.

Take solace in the fact that if you forget a line once, you will probably never forget that line ever again! Sometimes embarrassment is the strongest and toughest method of memorization!