Proportions
Some artists tend to overlook this part. Like when drawing a human figure, some mostly concentrate on the face and perfecting it first before proceeding with the other parts of the body. This will most often than not leave them with very little space for the rest of the body. This is why some drawings seem to have bubble heads. When you start your drawing, make sure your keep in mind the whole concept. Consider the space that it will occupy to be able to portray the whole concept. Cropping is part of drawing artistry, but there are proper ways of cropping your subject, in human figures for example, only crop the body at the waste, knees and ankles this will give you proper composition of your subject.
Heavy Guidelines
Guidelines are meant to be, well, guidelines. They should not appear on your final product. So make sure to make them just dark enough to be seen by you and light enough that it will not hinder the final outcome of your drawing. Try using hard pencils and using a very light lines.
Not Having a Goal
Before your start your drawing set a goal. And make sure as you go along, that you have your target in sight, don’t deviate, not too much at least. Some alterations from your original goal may be good for the final product.