If the customer coming is a regular or a loyal customer who has been with you for a long time, you need to show some recognition. Everyone loves recognition and for a person to know that they are valued in a place they frequently shop at is pretty flattering. Show some signs of recognition. Maybe try remembering the name of their kid, their dog, and mention it to them when they come in the next time.
You may be counting your cash, taking inventory, or writing a purchase order. But when a customer walks in, you need to pause it and pay attention to the customer. Ultimately, if there was no customer, you probably won’t have the cash or inventory. Aim to greet the customer within 30 seconds of entry by standing up, smiling, and using some words (good morning, how do you do, etc.). Ask them how you can help them today. When you’re done, you can resume whatever you were doing.