If one of your problems with the Olympic lifts is a weak back arch, that needs to become a focus for you in just about everything you do. Some of you have become very strong in round-backed postures and are finding yourselves unable to set a proper back arch in the pull of the snatch or clean, or even all the way through your squats. Reversing this can take a long time and a lot of patience and consistency.
One of my favorite exercises is the plain old back extension
(or hyperextension as we used to be allowed to call it). I think this is a great way to feel proper and complete forceful back extension because of the postioning (i.e. no competing tension), and it's an easy way to build up a large volume of repetition without killing yourself. This is an exercise that can be done every day before and/or after training.
Remember that we're talking about a back extension, not a hip extension—literally flex and extend your spine. Adding hip flexion and extension into the movement is fine, but you should be able to control your back directly and develop dynamic strength, not just isometric.
When you get to the point at which you're ready to add resistance, hold weight behind your neck instead of in front of your chest. This can be in the form of a dumbbell, but a barbell is a lot more comfortable. Have a pal hand it to you if needed, but you won't be using huge weights, so you should have no problem lifting it into position yourself. Holding it behind the neck allows you to get better and more consistent resistance with less actual weight--holding it in front makes it easier to let the weight move down toward your stomach, reducing the resistance and tricking you into believing your back is a lot stronger than it really is.
Isometric holds at the top of the extension reps are great too. I like doing a normal set of extensions with a very brief pause at the top, then finishing the final rep of each set with a near-max time hold.
If you're doing back extensions daily, modulate the volume and intensity somewhat day to day. That is, alternate between days on which you use resistance and days on which you do the reps unweighted. I prefer putting heavier/harder back work on training days that also have heavier/harder lifts and squats--nothing like going into a heavy lifting day with a tired back from the day before, although this is not entirely impossible or necessarily bad. If you have conditioned yourself properly, you should be able to manage.
I also really like good mornings with only very slight knee bend (really just unlocked, not bent) in your squat stance. However, these MUST be done with a complete and solid back arch, even if that initially means a limited range of motion. There is no point in just bending over with a round back a bunch of times--it will make your back strong, but not in the way we need it to be for the Olympic lifts. We're interested in specific postural strength. Over time, if done without sacrificing the arch, the good morning will serve as a hip extensor stretch as well as a back strength exercise. Focus on extending the entire length of the back--create an arch from your sacrum to the base of your skull.
In addition to developing better back extension strength, you need to be improving hip mobility to allow your back to extend in flexed-hip positions, e.g. squats and pulls. No amount of back strength will overcome really tight hip extensors. Make sure you're actually stretching your hip extensors and not further mobilizing your lower back. The best way to do this is always stretch with an arched back. A simple way to do this is to lie flat on your back with a rolled towel, ab mat or similar support under your lower back and stretch one leg at a time with the other remaining flat on the floor to help prevent the pelvis from rotating back and softening the lumbar curve.
With every exercise you do in training, if you should be arching your back (which is pretty much everthing but jerk-related exercises in weightlifting), you better be working on arching your back. Don't get lazy and think you're going to solve your problem by continuing to train the way you always have and throwing in a few sets of back extensions. In addition, force yourself to always be aware of your posture and your back arch. When you're sitting, standing, bending over to pick something up, work on arching your back properly. This is the kind of consistency that will really add up over time.