Don't be a slouch
Three cues I give multiple times every day during training sessions: breathe, abs in, shoulders down/stand up. In terms of slouching, however, standing up straight is not enough. You must build, stretch, and retrain your muscles to counteract slouching.
This month, focus on your posture:
1. Check yourself in the mirror. Are your shoulders and hips level. Do your shoulders tip forward? Does your head hang forward? Do you stand with knees locked?
2. Practice setting your shoulders by lifting your chest and allowing your shoulder blades to slide down your back. Open your rib cage to take a deep breath.
3. Notice your plumb line (side view): imaginary line from your ear lobe, to tip of shoulder, to center of hip/knee, to anterior ankle. Have a friend help you, if necessary.