I have a kind of order I work through, a lot of the steps have been mentioned above, but anyway:
Loitering round my local Waterstones. I live in a tiny city within walking distance of it, so am always in there and eagerly persure the new table sof books they have set out, and start to get a feel of what's just come out.
Then after I mentally bookmark a few titles I'll look them up on Amazon to see what the genearl consensus is. If it's 3.5 starts or less I sometimes discard the idea at this point, depending on why people don't like it. Sometimes I'll then but them, especially if I have had a leaf through and really like the authors style. I'm afraid I do buy most of my books on Amazon because it is so much cheaper, often £2.81 for a used book and between £3.85 and £6ish for new titles compared to the usual £8.99 or £7.99 that novels seem to be these days.
If I can't afford it right then then I add it to my Amazon saved for later basket so I don't forget.
I have a 'to read' list of books I have bought and really want to read next, the moment I have:
Mothers Who Can't Love - A Healing Guide For Daughters by Susan Forward which is my current read but it's so depressing that I am also reading
Bonjour Tristesse by Francoise Sagan
Next up I have
The Sex Lives Of English Women by Wendy Jones which I want to start before the MN author Q&A
Exposure by Helen Dunmore
All The Light We Cannot See by Anthony Doerr
Last two were a rare time I bought new books in a bookshop but they were buy one get one half price.
I mix up what I'm going to read next so I can change my mind and go with how I feel but I always have a few options to choose from, I read with an open mind and tend to finish books even if I dislike them. And I buy books I have heard about on Radio 4 as one person said, books people in the 50 books thread recommend, books friends recommend.
I also have an audible subscription which I keep threatening to cancel to they will offer me half price for 3 months or something which I have done a few times this year 
I also buy books on Amazon when they are on offer if it's one I've been meaning to read for a while. And I'm tending to read a mix of new books and classics/rereads of old beloved favourites this year. I haven't had any read duds... yet.