How to avoid blunders?
How to avoid blunders? Amazingly
Seriously though... Ask yourself before you play your move, what are they planning, what you are planning, and if the move that you are considering will hang a piece... Go to their side of the board and look at all their possible captures...
TameLava made some good points.
Before you make a move ask yourself what is your opponent threatening?
John Nunn introduced in one of his books the concept of LPDO (loose pieces drop off).
That means that your or your opponent's undefended pieces are often the basis of tactical combinations.
So next time you can look for undefended pieces in your or your opponent's position so that you can both identify threats or create/executes threats yourself.
Apart from that you should practice tactics regularly to avoid blunders. If you practice the patterns enough you will be able to spot them quickly in your own games. Knowing typical tactics and combinational patterns also helps you to calculate variations later on.
This should reduce "obvious" mistakes like 1-3 move blunders.
Not obvious mistakes are of strategic nature and require studying middlegames, chess strategy in general, pawn structures etc. But that's another story.
I'm rated 1400, and recently failed to pounce when my opponent left his queen hanging for a one move capture. This has happened a few times in recent history. Also, my opponent found a combo that could have won my rook. I would have resigned, but played on. At the last move, with my rook sitting there waiting to be taken, my opponent checked my king instead. I luckily won that game. And the computer usually finds 2 blunders in my games, and 2 inaccuracies.
The issue is there are many moves to consider, and not enough time to consider them during blitz. I doubt I'd drop pieces as much in long games. I'm thinking hard about how to grab squares and get a positional edge, and I fail to notice that my piece or pawn is hanging or that my opponents was. Give me a position and say, "Black to play and win", and I'll likely find it even if it is deep.