Every time the slave wants to turn his thoughts towards Allah, thoughts of other matters come sneaking in his mind. The shaytan is like a bandit lying in wait to launch an ambush, every time the slave wants to travel towards Allah, the shaytan wants to cut off his route. For this reason it was said to one of the salaf: ” The non-muslims say that they don`t suffer from the problem of vasvas. He said; ” They are speaking the truth, for what would the shaytan want with a house that is in ruins ? “.
This is a good analogy. It is as if there were three houses: the house of a king, filled with his treasure and savings, the house of the slave, containing his treasure and savings, and an empty house with nothing in it. If a thief comes to steal from one of the three houses, which one will he choose?
When the slave stands up to pray, the shaytan feels jealous of him, because he is standing in the greatest position, one that is closest to Allah and most annoying and grevious to shaytan. SO he tries to stop him from establishing prayer in the first place, then he continues trying to entice him and make him forget until he thinks of prayer as less important, so he starts to neglect it, and eventually gives it up altogether.
If the shaytan fails to achieve this, and thus the person ignores him and starts to pray, the enemy of Allah will come and try to distract him, by reminding him of things that he did not remember or think of before he started praying. A person may have forgotten about something altogether, but the shaytan will remind him of it when he starts praying. So as to distract him from his prayers and take him away form Allah, so that his heart is no longer in his prayers, and he will lose honour and reward from Allah turning toward him, which is only attained by the one whose heart is really in his prayer. Thus, he will finish his prayer no better off than when he started. With his burden of sins not reduced at all by the salah, because prayer only expiates for sins when it is done properly, with perfect khusho.
These are all extracts taken out of the book Al-Khushoo humility and devotion in prayer. IF you want some guidelines about how you can increase your khusho, i recommend you to buy the book at Darussalam. They still had a few copies left since i was at the site.