Beyond repetition, actually regurgitating, in your own words, what you've read is pretty helpful. I've found that, in doing summaries for my site Squeezed Books (http://www.squeezedbooks.com), that I've digested some things a lot more than if I had only read them. Anyone and everyone is welcome to try their hand at summarizing some business books for the site - absolutely free;-)

I've had some thoughts about ways to include essay/non book summaries, but so far I haven't bothered - stuff that's free on the internet is usually short enough to read, because it hasn't been inflated with fluff in order to make a book out of it.