Since I've had to refer to this a couple of times already and probably will have to again in the future, I thought I'd put it up as a post. These are some standard sketches I do for warmup and to practise essential skills.
Draw a lot of circles. Draw them as quickly as you can with as much energy as you can while trying to make it as much of a perfect circle as you can. If you keep drawing ovals, don't go slower, force yourself to counter-weigh the oval-ness until you get better circles.
Draw a lot of straight, parallel lines in multiple directions with varying pressure and distance. Again focus on making one, quick and straight motion with your arm to get the strokes down.
Draw S and C curves with varying widths, connecting them and combining them.
Draw ovals varying in width, height and angle.
Draw ovals using perspective guidelines. This is difficult since the shape is no longer symmetrical.
Draw isometric and one/two/three point-perspective cubes and boxes.
Draw cylinders and Spheres in perspective at various orientations and add in structural guidelines on top, learn to see how things get shorter or deform depending on the shape's surface.
Draw differing perspective grids to see how things align and relate
Draw winding and twisting scarves in perspective using as few disconnected strokes as possible, learn how things have to twist and deform to look realistic in space.
Draw composite structures in perspective including foreshortening, overlapping and all that crap.
I obviously don't do these every day, but I almost always start out with some quick circle and straight line practise to get warmed up. Doing all of these exercises regularly, especially when you're a beginner, is very important. Using these geometric practise tips improves both your mechanical skills to control your arm and hand, as well as your understanding of geometrical shapes and spatial relations.
