I don't have this problem. I've learned to say no quickly and effectively. It is all in the voice, the body language and the facial expression (usually one of pure hate does the job).
However, you do get those people who "attack" you and stick something on you before you have the chance to say no (happened to me a few times). Then I get out my wallet, dig out the smallest note I've got and give that. I do find hawkers etc popping up everywhere and getting more persistent and annoying!
e.g. in Menlyn there have been people selling Cape Town Fish Market stuff SINCE Easter. I can give them their entire pitch! Next time I go and they're still there I swear I'm going to tackle one of them...
