This usually comes into play when there is some sort of conflict between two desires, perhaps a long-term goal and a short-term goal which is contrary to it, such as a desire to eat and a desire to lose weight. In practice, I have found that which one wins out is a function not of some rational conscious decision about importance but instead a function of which concept is more strongly linked. Thus, if you want to lose weight but keep eating things in spite of yourself, you need to associate the desire to lose weight with as many daily, commonplace things as you can, so that the desire enters your mind frequently and becomes stronger.
There is a related case in which the conflict is not between two desires but between a perception and a desire. People are often put in the situation of wishing to believe that something is true yet having observations and data which indicate that it is not. Once again, the side which wins out will be determined not by rational thought but by conceptual strength. This is why cults try to become as central to their members' lives as possible - the more strongly linked the idea of cult membership is, the better they can compete with the person's observations that they're not actually a good thing.
We also find that people's concepts often gather into clusters - concepts in a certain cluster will be strongly linked to other concepts in the same cluster, but poorly linked to concepts in other clusters. This manifests itself as phenomena we often call "states of mind" or "moods" or "points of view." In extreme cases, it may even manifest itself as "multiple personalities".