When learning to draw, observing your subject accurately can be a challenge, because we are in the habit of making 'mental notes' about objects. There's too much information for our eyes to be able to take in and process everything, so our mind registers only the essentials. When drawing something familiar, it is harder, not easier, as your mind gets lazy. There are several approaches to trick your mind out of drawing what it 'knows' and into drawing what you really see. Click headings for more.
That is, look at the 'empty' shapes adjacent to the shape you are trying to draw. The shape between the nose and lip, or the space between the limbs of the tree. Practice doing some Negative Space drawing exercises so that you become used to observing Negative Space and can use it when working on a more important piece. Though it seems difficult at first, after a while, it becomes second nature.
Use the 'thumb and pencil' method of measuring the relative lengths of whatever you are trying to draw. You can take many small comparisons and draw guidelines to help you place features or objects. Click through to the next page of the linked article for more on using this method to judge angles.
Draw a grid of one-inch squares on white card and place behind your subject, or draw one on clear plastic to look through. Having precise reference points will help you make accurate judgements.
4) Drawing Upside-Down
Drawing upside-down makes everything look unfamiliar, drawing an inverted image forces you to look carefully. Try it as a warm-up exercise. Take a picture or photograph, and place it upside-down on your drawing paper. Copy it carefully - your picture is upside down too, of course - you don't have to mentally rotate it!
5) Check the Mirror Image
To double-check your accuracy, try using a mirror to view your drawing. Having everything flipped around makes it look unfamiliar, and errors in observation will be more obvious.