1) Ignoring: this works pretty well but encourages people to keep asking you until you're out of earshot2) Smiling and shaking my head: when I'm feeling friendly, I do this. Doesn't work so well.3) Saying "NO.": this seems to be most effective, although you have to say it LOUDLY and repeat yourself several times.
After a long walk to Chowatty Beach last night, I was feeling completely exhausted. Sweat was dripping down my back into the top of my pants, my hair was frizzling out every which way, and blisters were beginning to form on the soles of my dirt-encrusted feet. Soon I found myself accompanying my "NO!" with a horizontal slashing motion through the air. That, I soon discovered, was the best response yet.