Gestalt Principles are a great place to start when designing interactive interfaces. This article gives a brief over view of the main points.
It's always a good thing to refresh your memory even if you know them already - it can bring clarity to design when you are struggling with a tricky client - always go back to the basics and clarify your thoughts and whether they are working. Then you can present and defend your design in a clarified way.
Seeing is Not Believing What do designers need to know about perception? When we design for people, it’s useful to understand some of the processes that influence how people experience and perceive our designs. Perception is an individual and complex process, and how people perceive something will never be in our full control, but what we can do is optimize how we are designing to consider some of the psychological underpinnings of perception. The first thing to know is that seeing is not believing. There are many ways in which our perception, in particular our visual perception, can be tricked. You might recall the infuriating nature of optical illusions, which demonstrate this fact.