One of the best ways that I know of to ruin a drawing, painting, or sketch is to start it to tightly. How do I know? Because that's what I've done in the past, and to be truthful, I still catch myself doing it now and then. Looseness and flow, with practice these things can be achieved; the quickest way to get to a state of fluidity in drawing or painting is to just get out of your own way and let go. The idea of making something "perfect" is nothing more than being too much in your own head. By that I mean that as we are learning how to draw or paint we get too wrapped up in the idea of making the work look "right," but what does that mean? By what criteria are we judging what looks right and what doesn't? Obviously while learning a discipline a student should focus on the basics of whatever he or she is studying: form, value, light, negative/positive space, silhouette, color, texture, etc., but at some point there has to be a break with academic rigidity. So maybe the colors aren't exactly correct, and the form is a bit off. So what? Do you love what you do? Are you having FUN? Does your art feel like what you intended, and does it relay the information you want it to? If so then it's a successful piece in my opinion. So study, Draw from life (a lot) and follow the rules, then when you are ready...BREAK THEM.