My swim coach always used to tell us to always make sure to do a thorough warm up before going through with our training. This concept I think can be applied to art as well. Basically being able to just sketch a few pieces out just to get your mind ready for a long day of painting and doing concept art is a great way to improve. During my warm up art sessions I like to experiment and sketch things I wouldn't sketch take time to practice on things I'm not accustomed to drawing.
warm up sketch I did earlier
When I was a kid I would always here people say "You should always have a sketchbook with you" or "Doodle all the time to get better at drawing" The thing with just doodling I find, most people just end up drawing the same thing over and over again. Yes, repetition is a great way to improve and become a better artist. But the problem is they end up drawing the same thing over and over again then they end up becoming good at drawing that one thing. If you want to become a greater artist I suggest you DRAW don't just doodle. You need to go out of your comfort zone! DRAW EVERYTHING! Great practice I find is to just go sit on a bench in the park, city, subway and just sketch things whether it be landscapes, people, animals...Use the environment around as a teaching medium. Learn through your surroundings.