Okay, right now whatever you are doing, stop and think about your posture. Can you correct it and still do you job? Without thinking have you begun to slouch? For the rest of today, try to maintain good posture.
Get used to thinking about it and train your body to do it naturally. Like anything, if you practice it will get easier.
Whether you sit or stand, think of holding your head high, pulling your belly button in, keeping your shoulders back and down. Think tall. Think control.
Try not to let the workday destroy your posture and muscles. You are in control of your body.