It has been narrated from Salim b. 'Abdullah from b. 'Umar who said that the Messenger of Allah (S) told me: “Live in the world as if you are a stranger and just passing through and consider yourself as one of those who is already deceased. When you wake up in the morning, do not desire that you should live until the evening and when the evening comes do not think that you will be alive until the morning. Make use of your health before you fall ill, and make use of your youth before you turn old, and make use of your life before you die, since surely you do not know tomorrow (meaning on the Day of Resurrection) what group of people you will be gathered with.”[1]
This tradition discusses the most important issue of training (of the human soul) and that is the remembrance of death.
In the traditions, we see that it is been constantly mentioned that we must not have trust or reliance upon this passing world, and in Biharul Anwar it has been mentioned:

حُبُّ الدُّنْـيَا رَأْسُ كُلِّ خَطِيئَةٍ

“Love of the (material) world is the source of all sins.”[2]
It is the love of this world which is also brought about when a person is negligent of the transient nature and one thinks that this world shall remain forever. When a person engages in such a transaction (with this material world), then one will naturally develop a love and affection for it, and when this happens, it is very difficult to separate one's soul from the material world.
It is for this reason that one of the best ways of training a person and to combat against the performance of sins is to pay attention to death.[3]
A person must be such that when he wakes up in the morning, he does not assume that he will remain alive until the evening - and when he enters into the evening, he does not consider that he will remain alive until the next morning.
We should all live this state that we may not survive for another moment. It is this same issue - meaning the focus upon Allah (SwT) and turning away from the material world which leaves a deep effect on our souls. A person must truly believe that at any moment, one is in the grips of danger.
For example, pay attention to the heart which makes the blood reach all the parts of the body. The heart is the centre from which the blood is distributed, however it does not take its own nourishment from the blood which is inside itself. In reality, the heart considers itself as equal to all the other parts of the body (and thus waits for the blood to be distributed to it).
Sometimes the walls of the secondary blood veins become very narrow or develop a clot in them - and as it is known, blood can clot very easily. As soon as the blood comes out of the veins, we see that there is something inside it which makes it clot and sometimes it can also be seen that the agents which make up the blood can become contaminated and thus the blood clots right there in the veins and through this clot of the blood is so small - maybe one millimeter in size - however it takes over the channels in the blood vessels and thus, the blood can no longer reach to the heart.
When this occurs, then after a few minutes the heart will stop working and this is what is known as a heart attack.
When a specialist in this field (Physiologist) explains this to a person, that individual will develop fear since he knows that it is possible that he could die at any instant. Sometimes he may even plunge himself into doubts and the evil whisperings may overtake him.
The vessels that pass through the brain are much thinner than those which are contained in the petals of a flower and if there was to be any excess pressure on one of these, and if one of these blood veins was to burst, then with that blood spilling inside the brain, it is very well possible that the person could die.
In addition, it is possible that someone - due to having one bite of food to eat - could also die. When a person is eating, if a small piece of food or something else was to enter into the air-pipe which controls one's breathing, then the only way that this can come out is through coughing it out. However if the morsel of food or thing that went down the pipe was large, then it would stay in the pipe and there is nothing that could bring it out - except for death.
Thus in summary, dying is something that is simple and can come about very easily. Even in the trips we take we see that there are many dangers associated with them and even something like driving a car - even if we ourselves obey all the rules and regulations - it is possible that another person may not be following the rules and thus, due to him, the harm will reach you.
When a person knows that there is absolutely no guarantee in life and living, then why does one still go towards various types of sins? It is from this point that we must remember death and use this remembrance as the powerful breaks on our lives (like the breaks in a car) to prevent ourselves from committing sins.
When the blood is being transferred through our veins, we see that the small ones are right beside the large veins which come out of the heart and return back and feed the heart. Therefore, the heart is just like any other body part which is nourished through the tributary blood veins.
Thus it has been mentioned in the traditions that the most skillful person is one who remembers death the most. As well, in the traditions it has been mentioned that when Imam as-Sajjad (as) used to recite Suratul Hamd and reached the line:

مَالِكِ يَوْمِ الدِّينِ

“Master of the Day of Judgement”
he would continuously repeat this line in order to remind himself of the meaning of this verse. We see that the Imam (as) who possessed such a level of humility and infallibility acted in this way, so how should we who are polluted with sins - act? Thus, it is pertinent that a person reminds himself everyday of death!
It was once asked from Imam `Ali b. al-Husain as-Sajjad (as) what death was, to which he replied, “For the true believer, death is similar to a person removing dirty clothing from his body and removing the heavy chains from around a person and in their place, putting on the most beautiful clothes and the sweetest of perfumes and mounting upon the nicest of steeds and reaching the most appropriate of stations. As for the disbeliever, death is similar to removing his beautiful clothes and being clothed with the roughest clothing and being transferred from a place which he loves to a place which is filthy and being brought to a station which is full of fright and fear where the greatest of punishments will take place.”
In addition, when Imam Muhammad b. `Ali (as) was asked this same question, he replied, “Death is the same sleep that comes after you every night except that the time period of lasting is very long and the person will not wake up from this sleep until the Day of Resurrection.”
Even in Karbala on the day of `Ashura when the battle had become intense, Imam Husain b. `Ali (as) spoke some very beautiful words to his companions in relation to death: “Be proud O' you sons of noble men! Death is simply a bridge which will transfer you from a valley full of difficulties and troubles into a wide, spacious garden of paradise full of perpetual blessings. Which one of you would be upset at being transferred from a prison to a castle? However as for the enemies it is as if they are being moved from a palace to a prison full of punishment. My father, the Messenger of Allah (S), has said that the world is a prison for a true believer and paradise for a disbeliever and that death is a bridge for the true believer towards paradise while death is a bridge for the disbeliever towards the fire of hell.”

Tafsir-e-Namuna, vol. 22, pg. 362

Notes:
[1] Bihar al-Anwar, vol. 74, pg. 181
[2] Ibid., vol. 48, pg. 258
[3] From the point of view of the Qur’an, death is something which exists and is a transfer from one world to another world; thus in many verses of the Qur’an, it has been defined or explained as being “توفـى” in the meaning of taking something back - the taking of the soul from the body through the Angels. As well, in the Islamic traditions, there are also various words and expressions used in relation to the reality and certainty of death.