I've read in several places, here on BYC included, that overweight chickens and hens that start laying really young (per their breed) can be more prone to being egg bound.
Not much you can do about the age at which they start laying I guess, but maintaining them at a healthy weight couldn't hurt.
Even mineral oil will help, but fish oil is better. To check fat on a chicken, turn the bird upside down on your arm, like cradleing a baby. The soft part ahead of the vent is where fat can be seen. Separate the feathers and check it out. Fat looks like yellow streaks through the skin.