Just keep running the checklist of general principles through your mind every move, until they become automatic.
Is my opponent threatening something?
If I move a piece there can he take it?
Which pieces are currently undefended?
What is going on in the center?
And so on...
How deeply do you think I (approximately 1200) should be calculating before playing my move? Generally, I just consider all possible responses from my opponent without calculating deeper than that one ply. This seems insufficient, so I was wondering how many plies deep you would recommend someone at my rating level calculate before playing a move?
Thanks