Taken from Donald Normans Affordances and Design essay comes and excellent set of conventions to help guide the design of screen based products in a way that will help users understand what actions are possible, like most design conventions, each has both virtues and drawbacks:
1. Follow conventional usage, both in the choice of images and the allowable interactions.
Convention severely constrains creativity. Following convention may also violate intellectual property laws (hello Samsung et al). Sometimes we wish to introduce a new kind of action for which there are, as yet, no accepted conventions. On the whole, however, unless we follow the major conventions, we are doomed to fail.
2. Use words to describe the desired action.
This is, of course, why menus can be relatively easy to understand: the resulting action is described verbally. Words alone cannot solve the problem, for there still must be some way of knowing what action and where it is to be done. Words can also cause problems with international adoption. It is also the case that words are understood more quickly than graphics — even a well known, understood graphic. Words plus graphics are even more readily understood.
3. Use metaphor.
Metaphor is both useful and harmful. The problem with metaphor is that not all users may understand the point. Worse, they may take the metaphor too literally and try to do actions that were not intended. Still, this is one way of training users.
4. Follow a coherent conceptual model so that once part of the interface is learned, the same principles apply to other parts.
Coherent conceptual models are valuable and, in my opinion, necessary, but there still remains the bootstrapping problem; how does one learn the model in the first place?
Though written over twenty years ago the logic still is valid today, many of my chief complaints with iOS 7 could be solved by following the above. For complete detail refer to the original article.