Use the best sign-maker you can find and pay him more than anybody else. Plaster your windows with his signs, but not before you check his spelling. If someone is mad at you, they will tell everyone who will listen for as long as they are angry, maybe even longer. If they are satisfied, they have better things to talk about, such as who else they are mad at. So, correct any dissatisfaction, no matter how small and ask customers to send their friends.
Nothing beats word of mouth.
Treat your employees, and salespeople who sell to you, the same way you treat your customers. When filling a prescription, or doing anything else which could have terrible consequences if a mistake is made, don't make mistakes. Figure out a way to make it foolproof. With a prescription, the person filling it is never the one to put it away. The bottle is left on top of the prescription.