One of the biggest struggles in drawing or painting is when parts of our brain tell us what it "knows" something looks like, which prevents us from actually "seeing" what is in front of us. This results in drawing a more icon-like or familiar version of our subject instead of what is before us.
The Eyes Have It

An example that showcases this problem, one that I constantly come across, is in regards to drawing eyes. There is a part of our brain that likes to simplify things into icon-like images. It tells us that the eye is a pointy ended oval having a circle within a circle in the middle. Another part then tells us it is aware of certain physical details like eyelashes, which need to be added. The result is typically something like you see on the right.
Alternatively, we want to access those parts of our brain that are more observationally attuned to understanding what we are actually looking at. When this is applied we end up with an eye that is more accurate and tends to look more realistic, as seen here.
So how is this accomplished?
Dissociation From Our Subject
At first, this may sound counterproductive but to get around the problems discussed above of icons and information, we need to help our brains switch gears.
This is done through dissociation with our subject. It is the conscious and purposeful practice of looking at your subject in terms of measurements, comparisons, and angles. Keep from using the more familiar names or terms but look to the structures, shapes and relationships that are in front of you.
Talk to most portrait artists and they will tell you they are very conscious about NOT drawing or painting an eye, a nose or a mouth. They are more concentrated on the shapes, spaces, measures and angles that create their subject's unique features. Many say if they think of an "eye" or a "nose" they can easily create a more generic, familiar eye than the one they are looking at.
A Practical Example
In this case, what we want to do is dissociate with what we know about water bottles. Our brain has a simplified icon and will tell you all that it "knows" about bottles of this sort. At this point, we switch gears and purpose not to think of it as a water bottle but an object with no familiar descriptors. Rather we focus on its height-to-width relationship, what are the angles that point to its top? How wide is the green shape on top compared to the width of the object? Where is the halfway point? And so on.
Building Mental Muscles
So why this game? Once our brain becomes familiar with something, it doesn't need to remember the myriad of water bottles available on the market, but it just simplifies one and creates an icon for it. Our brain has many different functions and we need to learn how to access and flex the right mental muscles when it comes to observation.

Initially, We Always Observe
However, when we first observe a new subject or even view a familiar subject from a new aspect, we switch over to the observational functions of our brain.
To illustrate this I have included the same image of the bottle but rotated it 180 degrees. It is now unfamiliar, your brain doesn't usually see bottles in this orientation. Note your experience at seeing the rotated bottle. It is now more about shape and relationship than familiar icon simplification. You have dissociated from the familiar and are observing with more accuracy.
The more we choose to dissociate from our subjects in familiar terms and general identifiers, the better our drawings and paintings will be.