Whether you're selling new customers a product or service or making sure long-time customers are happy, being charismatic can increase their affinity for you and also make them more open to what you have to say.
Charismatic people share a common quality: They listen much more than they talk. When you're engaging with a customer, ask questions. Maintain eye contact. Smile. Frown. Nod. Respond--not so much verbally, but nonverbally. That's all it takes to show the other person they're important.
The key is to be personable while always having the customer’s wants and needs in mind, so you can have that balance of being both charismatic and professional.