Can a human being learn to ALWAYS be positive? Which practices create this mentality?

I don't think that it is possible to 'always' be positive because human beings are just not capable of such consistency.  Even the most positive person will hit a wall sometimes and not even want to try to see the good in it.  

But it is true that there are certain practices that we can use to train ourselves to look for the good in a misfortune or a challenge in a road block, or an opportunity in a threatning scenario.  

In fact, Islam teaches us this attitude and recommends that we remain patient in the face of adversity because the practice of patience can lead us to understanding the greater meaning behind the bad things that happen to us or others.  We can train ourselves to find solutions for our daily problems instead of wasting time grumbeling, or to practice changing our perception of what might otherwise be viewed as a negative by turning it into a positive.