If you can't be good, be careful.
Prov. If you are going to do immoral things, make sure they are not dangerous.; If you are going to do something immoral, make sure to keep it secret. (Sometimes used as a flippant way of saying good-bye.) Be a good girl on your vacation trip. Or if you can't be good, be careful. Ernest likes to close his letters with, "If you can't be good, be careful."