Getting an S
I don't know why there are so many QQ threads about this. I get an S just about every other game if not more and I am a shit player. I've gotten six or seven S+ as well on four different champs. I regularly play against opponents far better than me(at least better ranked anyway).
All I can say is, if at any point in the game you are wandering around aimlessly, you aren't going to get an S. Always be doing something or working towards a higher goal. If you are backing and have a trinket ward, place it before you go. It's little things like this that make you a better player. Getting the S shouldn't be your goal, it should be the result of your goals in game.