Romanian deadlifts (RDLs) are one of the best exercises for glutes and hamstrings. Trouble is, far too often they're butchered worse than a Christmas ham.
First, know that a RDL isn't a "stiff-legged deadlift." In a stiff-legged deadlift, you keep the knees locked out and bend forward like you're trying to touch your toes. However, if you do the exercise in this fashion – especially under heavy load – you're asking for trouble.
I cringe every time I see someone doing stiff-legged deadlifts and wish they'd be ditched in favor of the RDL – which is far safer for the lower back, not to mention a superior glute exercise.
The RDL is a hip hinge where you maintain a slight bend in your knees while pushing your butt back as far as you can. Lower down only as far as you can go and still maintain a flat back.
For best results, do your RDLs in the power rack with the pins set at the proper depth. This way you know just how low to go and don't have to worry about it throughout your set. Think of it like using a box for a depth gauge with squats.
Set the pins at a level that allows you to keep good spinal positioning. When in doubt, err on the side of the higher setting. You can always lower it as your flexibility improves.
This also forces you to control the eccentric (lowering) portion of the rep to avoid bouncing the bar off the pins. Place it down gently, pause for a second, and come back up.
With regular RDLs, it's easy to let your form deteriorate as the set goes on and start relying on momentum to help move the weight, but starting each rep from a dead stop reflexively teaches you to stay tight throughout, which protects the lower back.
You should initially reduce the weight as you adjust to the new technique, but it shouldn't take long before you're back up to using just as much weight as you could with regular RDLs – only now you'll be feeling them in all the right places, keeping yourself healthy, and using a full range of motion on every rep.