well, according to me, drawings are all about the shading. and the best way to learn shading is: greaseproof paper! the half-transparent paper you use on baking plates i used that when i was younger, put a sheet over drawings and photos in horse magazines and fact books and just followed the contures and shades. that way i also learned anatomics, proportions.
i still use this technique, i don't consider it cheating. it's very helpful to always keep my own drawing technique up to date when you feel more secure, use photos and carbon copy the contures, and then try to shade it yourself just by looking at the photo. (this was made that way) and then just start from scratch, both contures and shading yourself
practice, practice, practice is the main mantra. and to try different things, new things, every thing to draw, all the time