"Whoever Allah wants good for, He puts them to test. He puts them through difficulties." (Bukhari and Muslim)
Tests are generally of two types: Either a calamity / difficulty or when the dunya opens up to us in plenty. It is not how we react on a day to day basis where we see the true roots and honest depth of our emaan, rather is it when we are tested that the reality of our situation emerges.
There are certain acts that will decrease our emaan, and we will identify them here so that we can all inshaAllah recognize them and avoid them: (1) Excessive talking. As we know people will be thrown into the fire because of what they said in this life. (2) Looking at the forbidden. Allah tells His servents to lower their gaze in such situations. If something comes into our path of sight we can avert our look and do our best to avoid seeing such things. The Messenger of Allah, peace and blessings be upon him, said that "the eyes commit zina" reported in Ahmed. (3) Excessive eating. Our scholars say a person who eats too much because lazy and lathargic, which affects rememberance and worship of Allah. (4) Mixing with the wrong company. Those whom we spend time with infleunce us, so let us remain among the pious and righteous of people.
When asked: Who are the people most tested? The Prophet, peace be upon him, answered: "The Prophets, then the righteous people, and those who are nearest to them in perfection" (Bukhari)
"And most certainly shall We try you by means of danger, and hunger, and loss of worldly goods, of lives and of [labour's] fruits. But give glad tidings unto those who are patient in adversity" (2:155)