It is important to make the trial feel as if the lesson is tailored to them.
Find out about their background in martial arts.
Ask whether they have trained in a fighting system before, or if they are completely new.
Take into account that they may be nervous for the trial.
Start by explaining how Wing Chun is the only system in the world developed by a woman, and how being able to beat someone in a fight who is bigger and stronger than you applies to everybody.
Explain our strategy: fight on the blind side, don't fight force with force, watch the elbow etc.
Show how we punch with a vertical fist rather than a horizontal fist and explain why (knuckle alignment, body being behind the punch, ulnar and radius being parallel etc.).
Keep the techniques you show simple, easy and effective.
Defence against double arm grabs
Defence against straight punch
Defence against round punch
These are all good examples.
Avoid techniques that require too much footwork or clearing arms as they can be difficult as first techniques to learn.
Demonstrate how to do the same techniques on the wooden dummy.
After you have explained the strategy behind Wing Chun and shown the trial a couple of techniques, ask about what goals they want to accomplish. Ask whether they are looking for self defence, fitness, balance, flexibility, weight loss and so on.
You don't want to spend much more than about 30 minutes with a one on one trial lesson. 30 minutes is a good amount of time to explain what Wing Chun is about, show a couple of techniques and ask them about what they want to achieve through training.