I just heard a radio commercial for local jeweler. They were explaining that the cut of the diamond makes a big deal in the look of the diamond. They went on to invite people into their store to see a side by side demo of this they have available.
Professional photographers can use the same technique to demonstrate to clients the differences between professional and consumer photography and print quality.
Your basic display should have three prints.
1. Show a camera phone picture taken in the style as a consumer would take, no lighting, flat dull posing and poor clothing selections.
2. Take the same group, better clothing choices, adjust pose for better lighting, arrange group in pleasing composition. Large people in back or center, covered slightly with smaller people, you know the techniques. Have great color, tones, focus and definitely a top notch professional photo print on display next to the drug store made camera phone print.
3. Good idea to have your pro print also printed at drug store. Be sure the drug store does not do a great job.
When showing this display do not have a smug or condescending attitude as you point out the differences. Explain the clothing consultation was a discussion you had with your client and some suggestions that the family went back to their closet and made some additional choices. Do not suggest they had to go to a store and buy new clothes. Your amazing photos are the result of professional training, experience and taking the time to discuss some important details that contribute to make the very best memorable photograph that everyone is proud to own.
Get social media exposure telling everyone you have this available, be sure to mention the display during phone calls and always show and get comments to anyone visiting your studio.