Here's a simple thought: what are you looking at while you sew a seam? Don't look at the needle, or even the side of the foot near the needle. Look at the point just in front of the foot on its right side, the "right front corner" of the foot. I've found that, by concentrating on this point, it is much easier to get a consistent, straight seam. Our hands naturally move things toward what our eyes see. I discovered this while biking. If I am trying to avoid a pothole, or a rock, etc., I have to look at the path I need to take to avoid it. If I look at the obstacle, I will hit it every time.