1. "It's for safety." For some reason, my daughter wisely accepts safety as an absolute directive, so I invoke it whenever possible. For example, I characterized the "no slamming doors" rule as a safety rule, not a noise/behavior rule. "When people slam doors, eventually, people get their fingers smashed. So for safety, no slamming doors."
2. "That's just for decoration." We can walk into a store crammed with treats or gimcracks, and when she asks if she can get something, I just say sadly, "They're just for decoration, they're not for sale." She never questions this!
3. "The doctor says..." Invoking the authority of a doctor, dentist, teacher, or grandparent often makes a message acceptable. "The Yellow Room teachers say children must wear mittens to schools, not gloves." "I know you don't feel like brushing your teeth, but Dr. Smith says it's very important to brush every night." I'm not above pretending to send an email to get a particular answer.
4. "I know you know." My daughter hates being told "Don't," and she loves to show that she's a big girl. So I often say things like, "I know you know this, but other children don't know that you shouldn't tap on the glass of a fish tank. They don't know that the noise bothers the fish. Fortunately for the fish, you already know that."
5. "The sign says..." Like most children who can't yet read, my daughter is extremely impressed by the power of the written word. She will obey any sign. And because she can't read, a sign can say anything that I want it to say.