I listened to a farm radio program about this topic recently.
The tips presented were:
1) Make sure hens have plenty of fresh water. Eggs are 65% water. If you have water bowls that are freezing up or, as some do, are relying on snow to satisfy them in the winter, they can easily get dehydrated and hens will often turn to breaking eggs for moisture.
2) Plenty of comfy, soft, private nest boxes, so that they aren't breaking eggs in the process of laying them, either due to hard boxes or bickering over the best spot.
3) Up protein for sure...raw hamburger or ground turkey or fish meal are great "uppers" in the winter time, when their need for protein and fat is greatest.
4) Up calcium with oyster shell and/or yoghurt, etc.
5) Isolate the culprit(s) if at all possible for a while, as this may change their pecking order.
6) Add fake eggs to the nest.
7) Beaks can be trimmed or filed, just the pointy 1/4", to make it more difficult for them to crack the egg. (I've never done this and am not sure I'm comfortable with it, but that's what they said.)