Now about those blunders...
As in many other contests, even at the highest levels, victory in chess comes more often from mistakes than from great play. At the intermediate level and below, blunders such as missing a knight fork, are a fact of chess life. It's a good idea to implement a preventative strategy. I tell myself to make one last check before making my move - especially for captures, forks, checks and pawn advances, and to think one more time what is his best reply. This does not stop me from making blunders but it cuts down the incidence of the worst ones.
What about opponent blunders? These are gifts I gladly accept, but I tell myself never to expect mistakes from my opponent. In particular, I should not make moves for which I can see a refutation. Sometimes, it is tempting to lay a trap and hope for the best, especially if the game seems lost or hopelessly drawn.
By putting as much pressure on the opponent as possible I make it more likely that he will slip up and not make the best move. Some people don't have the patience to play careful defensive chess, and it is good to exploit this weakness. It can be a good idea to create opportunities for my opponent to blunder, so long as correct play on his part does not worsen my position.
I have noticed that making one blunder often increases, rather than decreases, the probability of another one. In such cases the position of the party being attacked just crumbles.
The biggest and most pervasive error in playing chess is simply not putting enough effort into playing for the other side. It's what I call "selfish chess". It's very easy to get caught up in my own plans, so much so that I only make a cursory calculation of the opponent's reply. I have missed mates in one due to this. Another way to see selfish chess is that it is playing against a static opponent, not against one who moves his pieces. The board cannot be frozen and each candidate move must be considered against a changing scenario of response moves.
Playing chess takes twice the effort that it superficially seems to require. I don't just need to work out my best strategy, tactics and the next move. I need to put in an equal amount of effort to do the same on my opponent's behalf. I have seen the enemy... and it is my own impatience.