I'm always looking for photo books- I just picked up on ebay the old twin palms edition of 108 portraits by Gus Van Sant. I remember when this came out I was not too bowled over by it, it seemed too easy- now I get how he really just had a moment with these people and put some polaroid to it. They are very natural which I guess I get now moreso with all this digital crap. Just what was in front of the camera which if it is good is good enough.
Another book I picked up was Thomas Struth Family portraits- again, just a very simple but highly descriptive set of pictures, perceptive.
How these help me is they keep me anchored to reality, so when I get a job I am NOT tempted to really load it up with "style" or stobist-ism, or any of that makeup.
Getting good subjects with good light in meaningful moments is enough much of the time.
I sometimes scour the internets, now, but used to be magazines and I kept tear sheets in a folder, ideas, looks, lighting, notes, just a reference to refer to looking for ideas. I still do that but I am careful not to look at it too much. After a while you see you tear out the same things anyway.
I think the best way to prepare is just to be observant and rich in your own life and associations, to always be making work for yourself, so that when something comes along you have an aha moment where a connection happens you don't expect- you connect with the subject on something unrelated, a book you are reading, a movie, etc. and that just leads to a better moment to make an image. Also to share something of yourself with the subject, not only do you not know them, but they don't know you- so sharing with them can help them trust you.