Drawing with Proportion
Whether you are drawing from an image or real life, measuring is key to an accurate and proportional drawing. Any measurement or angle can be related to another in the image and therefore in your drawing. If you want to scale your drawing proportionally, simply align the bottom right corner of your image and drawing; using a ruler, draw diagonal line that starts at the bottom left corner and ends at the top right of your image. With this diagonal line, the image can be infinitely scaled.
Value, Form, Edges
A large additive to a drawing's realism is its tone an accurate light and shadows. Pay close attention to the light and dark values and don't be afraid to push the shadows back!