Communicate clearly and directly. If the overarching rule is "Keep the classroom clean," make sure students know what that means. What expectations do you have for them? If the rule is about behaving appropriately, clarify what appropriate behavior looks like.
Use achievable actions instead of prohibitions as much as possible. For example, make the rule "Throw all garbage in the trash" rather than "Don't throw garbage on the floor."
Be brief. Though a general rule -- about appropriate behavior, for example -- may require discussion about what the rule includes, the rule itself should be a single sentence.
Reward students and classes where appropriate. Specify the punishments for breaking rules, but also specify the rewards for following them. This can range from praise and stickers to more lavish rewards. For example, consider hosting a pizza party if students throw all their garbage away for an entire month.