Get enough sleep at night, get enough food during meal times, then play in a quiet place where you can concentrate. Try to put any stressful events in your life out of your mind before the games if possible.
Warm up with some tactics and / or unrated games.
These things may seem minor, but they can make a big difference.
My blitz rating used to be somewhere in 1400s and one day all of a sudden I started losing every blitz match I played for the past 2 days making so many obvious blunders which I normally dont and my rating has now plummeted to 1140s. I'm losing to 1000 rated players whereas I had no problem beating those rated in the 1400s. Is there any psychological reason behind this? Is this common? I'm scared to even play another game as I'm afraid that I'll lose all of them.