Smile is the most important thing when you speak. Don't be like some people, who owe you millions. .
Polite language can treat customers as friends without being casual, take pains, be friendly and approachable, treat customers with praise if you can. If it doesn't suit him, don't be so straightforward, unlike some people, when you go to see that thing, the seller says it doesn't suit you!
Be positive, but not too annoying. Don't recommend it to people who are watching this carefully!
The most important thing is not to offend people and not to laugh! It's not that you offend people, this is the place to pay attention to. Once I gave a seller change, and he kept asking me for it. Don't be like these people.