Design expert Bruce “Tog” Tognazzini has a wonderful list of interaction design principles. It’s a long list, but it’s all worth reading. An example is his discussion of Fitts’s Law:
Fitts’s Law The time to acquire a target is a function of the distance to and size of the target.
From this law Tog draws the conclusion that important buttons should be bigger, less important buttons should be smaller. Items on the edge of the screen, or even better on the corners, are easier to access. Seems obvious when you read it, like all good design.