Improving the observation skill & making less blunders I've noticed that the most common reason for me to lose a game is not lack of knowledge but failure to observe whats happening on the board. This is specially true for rapid games but it happens in slow chess as well. In a recent game i moved a rook to a square where it was captured on the next move for free, although i spent several minutes before making that move! It also happens that i "forget" that a piece is defending something and move it away, or i don't see my opponents threat, even if it is straight forward. When doing tactics puzzles it happens sometimes that i look at it for several minutes and then suddenly realize something like "oh, i have a passed pawn in the top left corner on the board!"

If i could observe more quickly &amp; accurately and reduce the amount of gross blunders i would win a lot more games, specially rapid games. What do you do to improve your observation skill and blunder less?
I will go back to what Artur Yusupov, three-time World Championship Candidate, former top-10 player,  and one of the greatest living trainers recommends in the first book of his 10-volume series. In volume one, "Boost Your Chess - Build Up Your Chess", chapter 9, which is two-move mates, including composed problems:


  The aim of this lesson is to improve your calculation of short
  variations. It is more important to find a lot of options in the
  first few moves than to calculate moves long variations. Most mistakes occur in the early moves in variations. What
  is the use of calculating a long and correct variation if your
  opponent has a much better reply on move one?
  
  You must develop your skill at calculating short variations, while at the same time taking into account the possibilities available to
  your opponent. This skill should reduce blunders.
  
  Exercises with mate in two moves are very well suited for training in
  the art of calculating short variations with great accuracy.
  Emphasis as written, not mine.


I will take this one step further with my own suggestion. Try to train yourself that before you make every move, look at EVERY move your opponent has. I am not saying that you are calculating anything. You are simply looking at it so you consider it. It will do two things: If you are hanging a piece, just looking at it will usually "hit you in the face like a brick", and second, I have done many combination problems over the last 40 years, and sometimes, when I cannot figure it out, and look at the answer, I realize immediately that had I just LOOKED at the move, I would have seen the combination.

So, again, you are not trying to calculate, just consider every move. You will be amazed at what you register immediately.