The T/Th class made studies of rectangular forms (milk cartons, blocks) addressing proportions and diagonals. There are a few useful tips when trying to draw diagonals freehand (i.e. without using linear perspective). First, close one eye and hold out a straight edge (pencil, ruler, etc.) in front of you. Imagine you are resting the straight edge on the diagonal. Be sure to hold it as if against a window; don't point it. That angle should translate exactly to your drawing. Second, compare verticals, horizontals and diagonals to the edges of your paper (i.e. straight verticals should be parallel to the paper). And lastly, and possibly most important, try drawing the negative shapes between objects rather than the objects themselves.