When we perceive,we have the objects we Perceive and our Impressions of them in the Mind.
The Objects do not change from time to time.
It is our interpretation of them at a particular point of time changes.
So what needs to be changed is our perception of Objects.
However the information,Stimulii,which we have chosen to ignore,do not really go away.
They run in the background in our minds,much like the Andriod Applications that run in the background even when the screen is off.
In Android, you can force them to stop by going to your settings.
While Yama and Niyama indicate things one should bstain from and ones to follow, Asana speaks about postures Prathyahaara, Dhyana, Dharana explains the developments int the art of concentration.
These three terms, Prathyaahara, Dhyana and Dharana are different stages and many are not aware of the subtle, essential differences between them.
Prathyaahara is the withdrawal of senses and the focusing of the mind on the object.
Here one is aware that, apart from the object to be focussed upon, of the objects other than the object being concentrated upon.
Dhayana is that stage where the object and the one who is concentrating the object are present in the mind of the one who is concentrating.
In Dharana this duality of the Perceiver and the Perceived disappears.
This is the culminating step of Yoga.