Being accepting and supporting of an individual with a psychological condition is one of the best things one can do to assist them, especially since it provides a sense of belonging, safety, stability, and security that treatment, and even being an independent individual, cannot replicate or provide. Understanding mental health, especially the aspects related to the individual in question, can very much facilitate this, and be overwhelmingly appreciated (more information on relating to those with a psychological condition can be found on the Choosing to Share page).
Learning what one can about psychological conditions cannot take the place of having experienced them, but they can provide reference. One of the most difficult aspects of wanting to care for an individual with a psychological condition is that the experience cannot necessarily be related to; conditions affecting one's mind are a subjective and personal experience; while one may be able to readily compare the pain of a cut, burn, or broken bone with others, the same cannot be said for psychological symptoms. For example, depression is not the same as having a bad day, and even regarding depression, there are various forms and degrees, meaning that one individual's symptoms of depression can be quite different from those of another.
One source that can be a useful for discussing mental health, and conveying such symptoms to those without psychological conditions, is depressioncomix.com, a web comic whose creator, Clay Jonathan, experiences depression and uses his work to present various angles, perspectives, and situations related to mental health, including body image, depression, anxiety, and stigma.
Another way of explaining some of the difficulties of psychological conditions is "The Spoon Theory," a life story written by Christine Miserandino; the individual in question has lupus. As psychological conditions, lupus, diabetes, and many other medical conditions may not be easily noticed, they share the description of "invisible illness," and such can create a difficulty insofar as explaining the toll they take on an individual. The story provides a way of conveying how individuals with these types of conditions may have to go through much more, and plan farther ahead, in order to accomplish as much (or less) than an individual with mainstream health does in a day.