Keep them clear. If needed, add some clarity or likely exceptions or special circumstances to the end, but at least have the initial sentence contain the general thrust of it, to facilitate easier learning by the submissive.
Be consistent. Don’t make a Rule that you aren’t willing to enforce every time. Late at night, first thing in the morning, in public with friends or at home alone, if the Rule applies at all times, you must be ready and willing to enforce it at all times. To not do so would be to violate your side of the relationship, and weaken her trust in your discipline. If she messes up, she must be held accountable.
Write them down. Rules that can change suddenly upend the ordered world you are creating for yourself and your submissive. By all means change the rules, but write them down and discuss the changes with your submissive first so that you are both operating from the same page. Writing them down creates a strong sense of accountability.