You might compare zen to feng shui ("wind" and "water") - the Chinese art of balancing the different energies around us to create a healthy environment.
However, while feng shui has strict rules on how to organise space, zen decor is more unrestricted and modern.
Zen is all about minimalism. Indeed, zen (or zazen) is also a style of meditation developed in Japan that aims to pare down and simplify life as much as possible to focus on what is essential.
The same spirit is found in zen decor which generally features gentle and natural colours to soothe the senses : white, grey, beige and pale pink inspire calm and relaxation.
As far as possible, rooms are naturally lit. You can also play with spot lamps to add targeted subdued light interest.
For furniture, simple and clear lines are preferred. There is no excess ornamentation or complex detailing.
Natural materials are favoured (wood, stone).
There will not be too many ornaments on display to avoid cluttering the view and the space. Remember the aim of zen decor is to focus on what is essential.
Choose items with simple lines and place them carefully to bring balance and serenity to a room.
Finally, introduce natural elements like stone or plants which blend perfectly into this environment. They are a breath of fresh air and bring colour to your decor.