When a sales assistant asks "Can I help you?", can I respond with "No, thanks" ? Or are there more common ways of responding, instead of using "No, thanks" ?
You can use a positive sentence instead of a negative one. Examples are:
I'm fine, thank you.
I got it, thanks.
There are also responses that will signal to the salesperson that you will need their assistance soon:
Not at the moment, thanks.
If you are just browsing and not sure you want to buy anything you can say:
I'm just looking, thanks.
Those responses are more pleasant in my view, and they leave place to ask for help later.