Recommendation from friends? Reviews in newspapers/magazines? Seeing people read a certain book on your commute?
For me it's largely reviews in magazines or the weekend papers but I really find the Amazon (People who bought this book also bought...) function very influential. I also rely a lot on staff table picks in two of my most frequented book shops (Daunts and Waterstone's in Piccadilly).
Reviews usually - I just add everything that sounds interesting to my numerous Amazon wish lists and read the reviews to get a feel.
I flit around in Waterstones A LOT and just see if anything takes my fancy. I do sometimes go for film books which is how I discovered Louis de Bernieres (just from the hype of the film, I have not actually watched it - well half). The Notebook I also read this way.
Definitely the Amazon recommendations, plus magazine and newspaper reviews.(Daily Mail is quite good) If I like an author I will read all their books. I go the library every week and usually grab a couple from their quick choice shelves for a non taxing read.