Tactic puzzles, rated games against engines...because they tend to be very punitive (tactically). Also the time control of games you play is very decisive. If you don't have enough time to think and you're a 1800- player it's common to make blunders. So you need to play with slow time controls and check everything you can before making a move (i.e., never play by impulse).
Excercise against blunders?
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I have played for a while, but I still blunder way to often. I move pieces where they can be captured, move defenders, etc. What is the best way to improve in this area?