Arg, I just realized that I forgot to go to the first Love and Logic Study Group meeting this year! But seeing the reminder in my email caused me to read through some highlights on the basic principles of Love and Logic. If you aren't familiar, Love and Logic is a discipline philosophy that helps teachers (or parents) create firm boundaries while also helping kids foster good decision-making skills, self-regulation, learn to express themselves rationally and calmly, and take responsibility for their actions. At the recommendation of my assistant principal last year I started learning more about this approach, and eventually joined a study group to talk about how I was using Love and Logic in my classroom. I am not even remotely close to being an expert (one of the big no-nos is that you shouldn't be sarcastic, and I remember the second time I tried this approach, the student I was talking to eyed me suspiciously and asked, "Is this sarcasm or something?").
But I realized that there's one thing I've adopted and use regularly with a lot of success, which is, "Nice try!" Kids do stuff constantly to see what they can get away with. This used to (and occasionally still does) annoy me to no end: I would get so frustrated that kids were putting me in the position of having to tell them "no" all the time, or repeatedly doing things they knew they shouldn't. You absolutely need to communicate to them that what they're doing is not acceptable, but I've found that in most situations where a kid is just testing you, saying, "Nice try!" is a great way to communicate that. This tells them they're out of bounds with the extra bonus of showing them that you recognize their behavior isn't malicious, thus also reaffirming your faith in them and care for them. I love it.