Card counting.
Card counting isn't just for poker. Resource management is a vital part of the game. You want to be aware of what resources you have, what you're short of and plan ahead. Keep track of what resources are put into the game, and pay attention to the trades which are made and offered, these trades will tell you what your opponents strategies are.
With this information, you can identify when someone is close to building something they need and warn others from trading with them. You can also use the robber more effectilvey and make more informed trades, which is integral to the game.
While you do not have to keep track of every single card and resource each player has, you should at least have a general idea of what resources are currently available in the game and what's in short supply, and what advantages/disadvantages your opponents settlements put him in.