Draw the Reilly rhythms of the head.
Draw a photo upside-down.
Drape cloth over something, and watch and draw how it folds.
Find photos of or a model of a skull and draw skulls from many angles.
Find photos of humans or objects, and draw over these photos. Imagine and construct the underlying volumes.
Draw two parallel lines, and draw lines between them starting exactly on one line and ending exactly on the other line.
Find reference of a pose, and reference of a dressed person, and pose that person in that pose: combine two reference photos and turn it into something new.
Spend 10 minutes drawing something from reference. Then 5. Then 2. Then 1. Then 30 seconds. Become better and better at simplifying it.
Look at and copy paintings or movie still frames as quick thumbnails. Try to understand why their design works.
Rotate one box in space.
Draw cylinders rotated in space.
Draw ribbons that fold through space.