[stretchcat]

A calm tone won't necessarily make the cop calm, but a bold/agressive tone will almost certainly make the cop respond in kind. When I decline a request from a cop, I make sure to do it with a sugared tongue. The cop isn't entitled to the 'sorry' in "No, sorry", but that little indignity might make the difference so I'm willing to suffer it.

[hansvm]

I don't know if it's a good idea in general, but I've gotten good results from even more flowery language -- "I'm sure we'd be great friends in other circumstances, but while you're on duty I need a lawyer present for any further discussions." Anecdotally, not taking the effort to reaffirm that it's nothing against them personally tends to lead to a lot of pushback.

[stretchcat]

I've never tried going that far, but once a cop looked offended when I said he couldn't search my car, like I had impugned his honor, so I volunteered the explaination 'It's nothing personal, just a matter of principle." That seemed to satisfy him.