I normally don't say anything. Just make eye contact and shake my head with a polite smile while not breaking stride.

That's fine, or just say, "no thanks."

Depending on your mood at the time, and the aggressiveness of the request, I might just show them the "bras d'honneur". This is not seen much in the US, but it is understood, and it is a softer gesture than some others which can be used and is not likely to provoke an aggressive reaction. The true meaning is not lost, but the connotation is more like, "Get away from me with this (survey, pitch, etc.)"

I usually say "No thank you" and walk away. Be polite but firm.