>>6022994After your normal warm-up & reference workflow, draw similar stuff from memory, without looking at reference or your previous drawings. Then later on, or the next day, from imagination. Compare to the reference and see what you don't remember and study that more deeply and try again to draw from memory and imagination. You can also stare at reference for a couple minutes, then hide it and draw that from memory. Combined with constructive drawing techniques this means you fully understand the forms so you can doodle it more easily out of your head. Most of the artists I've heard that are good at doodling, say mileage is the most important. Animators and Comic artists who draw the same characters over and over can doodle them in their sleep.
Steven Zapata, Peter Han, ModernDayJames, Karl Kopinski, Kim Jung Gi and Glenn Vilppu have good videos/books on these subjects.
There are also automatic drawing techniques, where you don't have any expectations, so you can be surprised by the results.
https://www.youtube.com/watch?v=MJYGFwGhHnA 
