Ken Levine (TV writer including such things as MASH, Cheers, and Frasier) has a blog. A while back someone asked him about how actors memorize lines and since he wasn't an actor he polled actors he knows (unfortunately unidentified but I'm guessing many of them are well known).
It was very interesting to see the wide range of approaches and ease with which they do it. Here are the posts:
Part 1
Part 2
Part 3