The diet you feed, how much they eat at one time, and regular bathroom habits can effect this. I am assuming you feed a good diet, and that you feed two meals a day. Also that the dog goes at regular intervals- all this aside- some breeds can be prone to this, and usually expression monthly is suggested or bi monthly. However- this can be done incorrectly- and cause more problems than if nothing were done. If the whole sac is not "excreated" it will put whats left, and eventually impacted the gland. I usually recommend a vet or a groomer that knows how to make sure they get all the sac without brusing the dog.