My Theory of Blunders
In my practice blunders appear quite often. This becomes quite an annoying thing when you have played a very sound game defending or attacking, but in the end it goes down the drain because of a single blunder. There is a type of players that just love to set traps. One inaccurate "obvious" move - and you're finished.
I started to analyse how and when my blunders happen. It seems like I can classify the blunders in two major categories: Physical and technical. Although in a game they are mostly definitely tied up together, let me explain what I mean.
Physical side of blunders occurs when you have lack of concentration:
1) you are too relaxed (say when you are winning) while the opponent is fighting to survive
2) your thoughts are disrupted/distracted by something (sounds, bad thoughts etc.)
3) tiredness and as a result - apathy to the game (not sleeping well, illness, hunger etc.)
Technical side of blunders:
1) weak advantage realisation technique
2) weak tactical skills
3) laziness when counting variations
4) performing your plan without adapting to changes
So, how to make blunders disappear? I think, there is no magic, and everything is quite trivial.
The physical side of blunder prevention is merely to keep yourself healthy and fit, establish resistance to noises and disruptions, sleep well, do some physical exercises. The mood to play greatly depends on that. Remember that Botvinnik rested in various sanatoriums to train his body and spirit while preparing to matches and tournaments.
Technical side can be treated by solving tactics and practicing counting. And I mean not just like "ah, the rest is clear" but thoroughly counting the variations. The major disadvantage is inaccuracy of counting. The main motto is DON'T BE LAZY. In chess there is no place for "obvious clarity" unless it is proven over the board. Do not make an impulsive "obvious" move until you have checked that nothing is blundered - then proceed to move. Sometimes you literally need to sit on your hands to be patient. Spend more time but get it right.
Blundering is quite a persistent "disease" in chess. It doesn't disappear in one day unless you really start to work hard on your play. Truly, chess is the way to work not only on your play but also on your character improvement.