A 3D model which you can rotate and practice drawing from any angle. (Some models are parametrized and their proportions can be randomized.)
You usually don’t control the trigger. When it is dinner time, you feel hungry, and you eat. When it is time to go to bed, you brush your teeth. You can open your sketchbook and start drawing when you come home from school.
You must create a trigger when life is too busy, and you want to maintain a habit, and there are no triggers you can hook it to. For example, you can set your alarm for 5 AM to draw before the day starts.
But usually, the triggers are external, and you don’t control them much.
What you do control is the reward you crave before performing the habit and receive after. For example, you choose the toothpaste that has the nicest taste.
Find a way to be happy with your art, and you will crave making more art every day.
This is a problem for us artists. We see the mistakes in our work and often don’t like how the results come out, and that is not much of a reward.
I currently draw this way: I lay down the broad shapes with an unsharpened green color pencil to get the proportions and such right. Then I do a tight drawing with a blue technical pencil. Then I ink. I like the way the sketchbook pages turn out.
Find a way to end up with something satisfying at the end of a drawing or painting session or, alternatively, a bit harder for visual artists: find a way to enjoy the process without considering the ouput. Both can serve as the reward you crave the next day.