Do Not Trust the User's Short-Term Memory
People don't like to remember a lot of little details, and certainly don't like being forced to do so to be able to complete the task at hand. Make all the information needed to complete a form or make the right choice in a dialog easily available to the user, but don't overdo it with irrelevant information.
Support Undo
Let the user have the possibility to undo the actions he performs as much as possible. There is no reason not to, and the user will feel more confident and more efficient.
Gestalt Principles
The Gestalt principles describe phenomena related to the visual perception of things, and are necessary to keep in mind when designing user interfaces. The way users' minds map various elements onto well-known things affects how they perceive the functionality of these elements.
Similarity
This effect occurs when various elements largely look alike and is thus perceived as a single unit. This can be used to express the fact that these elements belong together, and perform related tasks. This applies to both an elements shape and its color.
This effect can also be used in its inverse, using dissimilarity to emphasize a certain element. A common use is having the currently selected section be marked with a different color than the rest.
Proximity and Grouping
If elements are placed near each other, they will be perceived as a group, and one might guess that these elements are related in some way. Thus, one must always be aware of the spacing between elements to make sure no false relations are suggested, and that the relations that do exist are made clear.
One can also add clear separation of elements in form of borders and different background colors to establish the distinction between groups.
Continuation
This effect occurs when a shape compels the eye to move through and continue to another shape. Curves and lines are effective this way, and can be used to guide the user's attention to a desired area. Use this to emphasize a relevant or important element the user should notice immediately, such as a next-button that would otherwise be unnoticed in favor of other design elements.
Closure
This effect occurs when a shape that is not complete is composed in such a way that the user will complete the shape in their mind. Consider as an example that ¾ of a circle's outline would be enough to suggest the complete circle as a shape.
Affordance
Affordance is the term that describes the effect when an object strongly suggests a specific action, such as a door handle that says “push me down”, and the handle of a sword that says “grab me” (and the blade that says the opposite).
An object that suggests a certain action very strongly is said to have high affordance, and an object that doesn't has low affordance. An example of the two would be an ordinary door handle that must be pushed down to open the door, versus a door handle that must be pushed in and then lifted up to open the door.
The level of affordance depends on the person who looks at the object. Someone who've never seen a door handle might not know that it must be pushed down to open the door, while it would be obvious to anyone else.
Thus, you should be aware of what the elements in your user interface signal to your users. If it looks like it can be clicked, it really should be.
Conclusion
Hopefully, you have been enlightened somewhat by this article and can now go over your interface designs and see all the things that don't work out, and make the necessary changes.
Always consider how the potential user will perceive your interface, and remember that the potential user will rarely have the same notion of the world as you do. The things that are obvious to you might not be so obvious to everybody else.
Even though the theory is necessary to know, and will save you a lot of trouble, don't forget the value of having actual users try out your interface and see how well they adapt. You will more often than not learn some surprising things about your design.
