This is an easy concept to understand, but one that is very difficult to practice.  No one likes to think of positive things when undergoing adverse circumstances.  However, I have found that this is a "trainable" thought-skill, but it requires a lot of self-discipline and honesty.  

1) The hardest part of this process is to remove the negative emotional reaction we naturally display when stress increases and problems arise.  It's difficult because it is a natural reaction, and to overcome a natural reaction means you must do something that your human nature feels is unnatural.  This is where the discipline and honesty come in. 

2) You have to be honest enough with yourself to admit your frustration at the bad circumstance and the negative emotions,

3) but then you also need to exercise the discipline to force yourself to have a positive attitude; and I do mean force yourself.  

I know it sounds crazy, but once you follow these three steps, your emotions and outward demeanor will follow your attitude!  It doesn't matter if your attitude is forced or not; your emotions and demeanor can't make that distinction, and they must simply follow the direction of the attitude!  Crazy, but it works!  You will then not only have the clarity of thought to recognize an opportunity that may have been missed due to your previously embroiled emotional state, but you will also begin to train your thought patterns to follow more positive lines.  The more you practice it, the more of a habit it becomes.  

The more opportunities you are able to recognize and follow up on, the more positive events and circumstances ensue, and you will have been successful in creating a more healthy, and enjoyable, cycle of positivity.  Congratulations!

(Now go do it!)