How to perform this exercise:

Concentrate on some object and try to notice as many details as you can.
Close your eyes and try to picture the object that you just looked at, in your mind.
You will most likely recognize many details on that object that you have missed.
Open your eyes and notice all the details that you had missed.
Close your eyes again and try to envision the object, see if the image in your mind becomes clearer.
Do it several times and you will realize that you are getting better each time.