The advice to "never be first to state a number" should really be "never divulge your current/previous salary".
Anyhow, given that, there are numerous options to decline to damage your own negotiating position. My personal favorite is "As a matter of professional courtesy, I decline to comment on the specific policies of past employers. Don't worry, I will treat your confidences with the same level of professionalism when someone asks me about them in a few years."
It is unfalsifiable (as opposed to what people sometimes suggest on HN: "That is under NDA", which is a claim that can actually be a lie), conveys status, and (gently) informs the person you're speaking with that you're aware how the game is played.
I can't believe how stupid I've been all these years :( I'll admit that when recruiters have asked me this question, I've behaved almost like a deer in headlights.
My current job, I accepted the offer as is. THEN asked them if they had any salary negotiation room. Of course, I was told no. I felt like hitting my head against a wall right after that phone call. I won't be that big of an idiot again!