Think of sitting tall all the times, concentrate on pulling your navel into your spine.
The majority of the power comes from the legs, ensure you are initiating the Drive and pushing firmly away with your legs on each stroke.
Focus on having a strong pull of the handle towards the body at the end of the drive phase.
Try to get full compression of the legs and full reach of the upper back and arms to a position comfortable for you. Stroke length will vary according to your flexibility and body shape and should improve over time.