How does Death of a Salesman relate to real life?

In an interview, Arthur Miller spoke of an audience's reaction to Willy.  His words prove the relevance of the drama to real life as he described how the audience reacted:

[The audience members] were weeping because the central matrix of this play is ... what most people are up against in their lives.... they were seeing themselves, not because Willy is a salesman, but the situation in which he stood and to which he was reacting, and which was reacting against him, was probably the central situation of contemporary civilization. It is that we are struggling with forces that are far greater than we can handle, with no equipment to make anything mean anything.

In the end, the conception of the American Dream as one where there is financial security and emotional happiness as a result does not speak for the vast majority of modern Americans.  In a setting where financial insecurity is more present now than at any other time in recent history, at a time when consumer confidence is one of its lowest, and at a point where there is more fear about the future than optimism about it, Willy's predicament is more relevant now than ever before.  The idea behind the recent Occupy Wall Street movements is that "We are the 99%."  Willy's plight is this 99%.  His condition is one that speaks for more people than anything else.  His condition is one whereby that individuals do feel that they must do what they need to find financial security and equate monetary success to emotional happiness.  The need to sacrifice all upon the altar of material wealth becomes one of the most profound revelations that Willy experiences and one that is seen in the modern setting.  It is here whereby the play and drama relates to real life.

How does the past tragically impact the present in Death of a Salesman?

In Death of a Salesman the past continuously impinges upon the present. This is because the entire play is based on Willy Loman's final reckoning; the last episode of his long life of mistakes, where he finally comes to terms with what he has done, and makes his first (and last)  attempt to redeem himself. 

Willy's life story comes to us as flashbacks. Since Willy is a sickly, tired, aging man, these flashbacks from life serve as a way to tell the audience that life is, literally, "flashing in front of his eyes". This concept is often attributed to an imminent death, which is what ends up happening in the end. 

Aside from presenting itself in the form of flashbacks, the past has also made a massive dent in Willy's life. It is because of his past that Willy's life is as empty, shallow, unfruitful, and inconsequential as it is now. 

This is why.

Willy had choices that he did not want to make. He had many opportunities to join his brother, Ben, and go get the riches that Ben had acquired by going to the mines. Willy, ever second guessing himself, chose to listen to his wife Linda and decline the invitation. However, now in his twilight years, this indecision comes back to haunt him. What if he had taken the risk? Would he be as hard for money as he is now, in his sixties? Would life have been easier?

Willy's bad choice-making also comes in the form of the career choice that he picks. Willy does not like selling; otherwise he would be good at it. He claims to be a good salesman, but he is probably quite average. Regardless, the only reason why Willy becomes a salesman is to follow the steps of another salesman named Dave Singleman, a myth of a man whose funeral was attended by everyone.

The problem is that this is like chasing a waterfall: no one must plan a life based on the life of someone else. The path of each individual must be walked alone. Willy's choice to follow the life of Singleman leaves him in a job in which he does not succeed nor excel. He does not even leave anything for his family to reap any benefits from. 

The past also comes to haunt the present in the form of Biff's unproductive life. His once popular and successful life as a potential football player comes crashing down when he accidentally discovers that Willy had a mistress. While Willy continuously blocked this incident, and blamed Biff for his own problems, it is clear that this crass mistake on Willy's part definitely ended up hurting the entire family, altogether. 

In all, Willy's past demons come to haunt him now that he is in his very last days of life. Perhaps these flashback serve as one last attempt to ask Willy to come out and redeem himself before the end.