I really, really REALLY hate it when people write in books. Possibly I'm a book snob, but I just love opening a book, no matter how old and only reading what the writer has written. (And not finding awful dog-eared pages, Is it that hard to find a piece of paper to slip between the pages? Respect for books!!!)
But the problem is that I often love a particular sentence, or phrase and when I go searching for it again I can never find it.
So this is a tip I got from somewhere. Get large post-it notes and stick one on the inside of back cover. Then when you are reading and come across something you want to remember you just go to the back of the book, write down the page number and either the quote or the main idea. (Please, please don't just write it in the back cover, I beg you!)Then later when you are looking for something you remeber reading just turn to the post-it and read the notes to find the page the quote/passage is on.