I agree with TurboJ, keep it simple.
I used to do a lot of trial lesons when i started instructing, and i was brieffed by the more experienced fellows at the school.
What we use to do is take a model aircraft (the ones pooleys sell made of wood with moving surfaces where you can see stick and pedals on the outside of the model) and explain the effects of the controls in a very visual way, and writing down on the board as we explained it but keeping it very very simple.
later, in other schools i had to do it with no model, and i used to go near the real aircraft and do the brieffing by moving the surfaces and controls for them to picture what i was saying, keeping technical jargon to the minimum.
beware, in some countries (UK for instance) the authorities will not appreciate if schools do trial lesons with no brieffing, legally speaking it cannot be considered a trial leson if you don't brieff them properly.