When you are drawing a sketch try to work very fast and without any measuring. Your subjective impression of a model is really important, it's even more important than authenticity in some case. You can make linear, tonal and combined types of sketches with various styles and tasks in mind. Identify your points of interest, if you are drawing a human it can be a face, hands, and feet. for example. Draw more, create some challenges for yourself (draw only in tone, with eyes closed, without taking a hand off the paper, etc.). And also really important - have fun!
