Stand openly, welcoming them, inquiring of their needs and showing readiness to listen.
In talking with them, generally be clear and assertive, neither dominating nor submitting, although some movement in either direction can work if done with care and in the right circumstances, for example being as servile as your context demands.
Clear body language is relaxed and precise, without being held in or under control. It supports speech, for example in emphasizing key points, and never contradicts it. It does not include unnecessary movement which may cause confusion or betray nervousness.
Match and move
Start off largely reflecting them back, for example by matching body language and using similar verbal style, in order to create an emotional bond with them. Keep your body at the same level as them, for example standing up to greet them and sitting at the same time (not slumping below them).
Then move or speak differently. If they have bonded with you, then they will now be following you, rather than you following them. If they do not do so, then keep matching and moving until they do. Do not move to closure until they are following you.
When you lean in towards the customer, you are getting closer to them, creating a bond. There is a danger here in that the person may consider it an invasion of their personal body space, so still treat distance with care until you have their confidence. Leaning in, rather than moving in, is a tentative action and hence is more acceptable.
Leaning in can be conspiratorial when the sales person lowers their voice, as if making this exclusive offer in a way that cannot be heard by their boss or other customers (who would of course snap it up immediately).
Leaning also lowers the body and is a subtle bow, showing respect and deference, hence taking the sting out of any perception of dominance.