Remember that the RDL is different than a conventional deadlift. It's all about placing tension on the hamstrings and disengaging the quads. So remember these two cues:
First slightly bend the knees. For the RDL, the knees should be "soft" and not totally straight. Now bend down or hinge, but keep those hips high.
Use this cue during the lowering phase of the lift. Don't just let gravity pull the bar down; actively push it there, then push your hips forward and squeeze the glutes. This will help you to keep your hamstrings tense throughout the lift.
Too much knee-bend. This will make the RDL less effective by causing your hamstrings to lose tension.