Here are some quick drawing exercises that can help with certain aspects of drawing:

Comparative Proportions

Draw something large next to something small, simple objects are best.

Why does it work?  It forces us to make comparisons between two things which makes us look, double check and ‘measure’ making comparative proportions.

Tone 

Draw an object under a lamp.

Why does it work? Because it makes us see where the light source comes from and how it behaves when it hits the object.  A strong light makes extremes of light and dark which makes tone easier to study and forces us to use all the shades of tone.

360 degree drawings 

Draw an object from 4 different viewpoints by turning it around in a circle.

Why does it work? Because it makes use see the 3 dimensional nature of the object and forces us to see how the object changes at each turn.

Speed drawings

Set your timer and draw as many objects as you can in five minutes or ten minutes if you prefer.

Why does it work?  It forces us to make quick observations and get the main essence of the object or subject down.  It loosens us up, makes us less perfectionistic and ‘warms us up’ for more developed drawing.

Go on, give these a try!