The Romanian Deadlift is one of my favorite exercises for not only strengthening but lengthening the posterior chain. By lengthening, I mean increasing mobility and dynamic flexibility of the hamstring and low back.
The first way to improve the RDL is to make sure you are going through a full range of motion.
Sometimes I have beginners go through a shorter range of motion (just below knees) before they learn to drive their hips back (hip hinge) and keep their back flat. But as they continue to be able to control this movement, I have them increase their range of motion (plates to the floor).
Shortening your RDL's, like any other exercise, is just failing to strengthen and lengthen those muscles through a full range of motion. Start with lighter weight and learn to control this movement through a full range. Once you get the technique down, you'll notice that by adding weight can actually "auto correct" your form and improve that loaded stretch.
Second, learn first to shift your hips back, then your knees, to clear space for the bar. When an RDL is done correctly, the bar path is straight down. This happens because the butt is driven back into a hinge, and while the knees are only slightly bent, the posterior weight shift of the body will push the knees back to clear space for the bar.
Too many times, I see people bend the knees and then try to drive the hips back. This not only kills the hip hinge but forces the person move the bar forward as they are moving towards the ground. Not good for the back, and does nothing for the hamstrings and glutes.
Third, finish the movement. Upon bringing the bar back to waist level, finish the movement by driving your hips through the bar (by squeezing your glutes) and pulling your shoulders back. This ensures you are completing the exercise by properly firing in the hamstrings and glutes