1. The physical product - it is actually a good idea to take a simple piece (you can just take a piece of wood like the inventor of the Palm Pilot did) and do an improvisation theater with it. For this you explain to a user what it does and hand it over to them and ask them to explain to you how they use it.

2. The visual mockup: For me the best results have been to do a simple 3D model and render it with Keyshot - there are rendering alternatives called Bunkshot and some others. Basically these are 3D Rendering programs that very easily allow you to assign materials to a product and create a high quality render very quickly without having the very complex and advanced knowledge required to do a VRAY render. Regarding the 3D model itself a good start is Sketchup, formerly owned by Google.