Keep a sketch book and set of pencils on hand. Every day sketch something from observation. These are some subjects we have studied and sketched by observation:
The way the light falls on a tree at different times during the day, sketching the tree multiple times as the sun arches across the sky.
The same object from different angles. The cow from front, side, and back views.
A scene from three different distances, close, distant, and very distant.
Herbs, plants, trees, up close with details to help you remember the physical attributes of the flora.
Drawing the same object in different media. For instance drawing a piece of fruit multiple times with either a pencil, charcoal, pen, paint, chalk, or finger paints.