No eye contact is key in these situations. It's even easier if you have headphones and can pretend to not hear them.
I typically just say "no thanks" and keep walking if they get my attention somehow.
I have a personal policy for stuff like this. If I am ever bothered, called at home, someone comes to my door, etc. looking for money or similar, they automatically don't get it. I donate to charities when I can, but strictly those who do not annoy me constantly with phone calls, etc. So the same thing applies to these salespeople in airports and such. Their willingness to constantly annoy people and guilt them into giving them their attention guarantees their company will never see my business.
The worst is when charities call though, they start with a high dollar amount, start working down a scripted list, and they eventually get to like $10 or something and are like "sir, are you telling me that (Insert cause here) isn't even worth $10 to you?". That's when I hang up, and donate to a different charity.