1. Check your grip and make sure it is not rolled to far clockwise, or to the right for a right handed golfer.
2. If your posture is very erect from the hips up bend more forward as this will make your back swing more vertical.
3. Be sure that you are maintaining the "Y" formed by the two arms and the club shaft as you execute the back swing. This will keep the club head more in front of your chest at the top.
4.Lastly, at hip high, both elbows should point at the ground with the right slightly higher than the left.

If you make these adjustments, your flat swing will disappear.
Cheers,
Bob