A teacher suggested that the word "but" should not be used when giving feedback to children about their work, as it negated any of the preceding praise. Also, rather than just criticising, specific comments should be made as to how improvements could be made, e.g.
"That was a very good piece of writing with lots of interesting information BUT you've made a few spelling mistakes."
She suggested:
"That was a very good piece of writing with lots of interesting information. There are a few spelling mistakes - it would help if you use a dictionary when you're unsure about a spelling."
Presumably this advice might work just as well for behaviour issues.
I'm sure most of us have experienced the negative power of "but"!!