No takers?

I'm guessing the muscles you have to strengthen here are your cervical extensor muscles. Usually if there is a muscle imbalance you have to loosen one muscle, but strengthen the other. In this case, loosen scalenes and tighten the extensors.

I think I'm not the only one who has noticed that loosening the scalenes is no small feat. But maybe if you strengthen the extensors it becomes easier.

One exercise I've read for this is to lie prone on your stomach, keep the head neutral, lift your forehead off the floor by an inch and hold for a few seconds. Repeat.

Going to try it today.