The big enemy of stag is sudden changes in temperature and/or humidity. This can be a result of going outdoors and then back indoors in the winter, and also from the dry winter air in your house when the furnace is running.
This is less of a problem with stag than with bone and ivory, but can still happen. Coating the stag with a light coat of mineral oil three or four times a year will help, though how much it will help is debatable.
Mineral oil will yellow the stag a little, but that's going to happen with age anyway. And it's about the best thing you can do to ward off cracking.
That said, as a good friend of mine has said in the past about all natural handles, "it's not a matter of if it will crack, but when."
Good luck!