Patience and a smile for the employees do go a long way for retail and service industries.
When I was in the grooming salon, our store would be booked up 5-6 weeks in advance. People would scream profanities at us, we were ruining *insert holiday here* because they just realized that their dog was a matted mess, and how could they possibly have a dirty dog running around when Great Aunt Ida was going to be here? The same customers did this every year, for at least the four years we had the computer system to keep notes in.
The ones who booked early, who recognized that we didn't have any openings for their preferred times, and who were just nice about us being full were usually the first ones called off the waiting list. And the customers who we knew were more willing to be flexible with us and leave their dog a bit longer were also squeezed in.
When I'm shopping, I try to be patient with everyone and smile at the employees, and thank them for helping me. Yes, it's their job, but sometimes a kind word can make the day that much better.