Saying no in Japanese

It's hard to say no clearly in Japanese without offending. I still remember a Dutch mum in a playground when she was still new in the country who was incensed that her son was being (mildly) teased by some other kids. She demanded I tell her how to say no to them. It was hard to give her a good answer, I was only a couple of years ahead of her. Now I would say "Da-mei, with your forearms crossed in front of your chest."

This post explains some of the ways to say no in Japanese.

I commonly come across this problem at the grocery store when they offer me a bag and I have my own or if they try to get me to sign up for a store points card.

 I often use yoroshi desu in these cases. Which, I'm pretty sure means "I'm okay thanks", please correct my Japanese if you know better. Of course you can use it in a positive sense too, in that "Is this okay?" So there is still potential for confusion, but it usually works.