I have been demonstrating for many years, the thing that I am asked for the most is bread and butter advice, how to fit a form properly, how to hold a file, how to apply just the right amount, buffing in tips and applying sculptures that look like tips.
Most companies that you demonstrate for want you to sell something, the biggest market and markup is liquid and powder and will lead in to file, polish, tip, adhesive, forms and accessories sales.
Nail art is quite a niche market, particularly 3D, sure, it's great for a pose, but it doesn't sell product.
My favorite demonstration (I will only do one nail and expect them to get their other nails done at other booths, I'm not there to work my ass of doing free full sets) is to do the most perfect and most natural looking nail possible. Custom blend, muted natural looking whites (many have lost touch as to what looks real), in thickness, color, length and shape.
There is nothing quite like someone coming back at the end of the day with a hand full of lumpy artificial tragic looking fake claws and your nail looks absolutely beautiful and by far the best.
I also sign my name on the finger, so people can see when they are at other booths getting a nail done who has done the best looking nail.