Keep a list of the titles you read
Write notes while you read
I like to highlight, underline and write notes when I read things I know I'll want to remember. Most e-book apps will let you highlight, bookmark and take notes. I write in my books too, but if I don't want to I write in a notebook (sometimes I rip out the pages and stuff them into the book I read, other times I can't or don't need to do that and I just keep them in the notebook. Spiral binding is useful for this. Then I can decide what to do when I'm done reading). I do not write long notes (unless I do), and I'm not a collector of quotes (though I'll underline particularly nice ones anyway) so often it is just a page of random thoughts or guesses on how the story progresses. I usually pair my notebook note with a page number so I know what I'm talking about if I need to reference it later.
Write down your thoughts afterwards
I know, you'd rather start the next book, hang out with some people you like, catch up on twitter or watch a TV-show/movie, but taking ten minutes to think about what you just read and then write it down will make it easier to remember later. I don't always do this, but when I don't I often regret it. I write my thoughts in a variety of places: in a notebook (I keep it next to where I usually read), in the review section of my Goodreads account, here on the blog (or on my Norwegian blog), and sometimes in some document or note-taking app on any of my digital devices (Mostly I use Evernote for this). This way I can read over the paragraph (or three) that I wrote and remember things. If I am reading a series I'll also write down things I'll want to keep in mind for later or questions I have. My personal notes are sometimes very confusing and I forgot what I meant, so I recommenced full sentences, but sometimes just keywords work for me, or lists of things I liked and didn't like. This is very much the same method I used when I read my curriculum in school, and it works quite well. If I can be bothered I read over it, and perhaps add new notes, about a week later too, but I'm mostly too lazy for that. If I borrowed the book I sometimes take a picture of the cover and keep it somewhere as a bonus visual reminder.