Before I started using watercolor a whole bunch, I did a lot of sketching with a pen, because I figured if I wasn't allowed to erase I'd learn to be a little more bold and decisive with my movements, and I concentrated on working fast and loose. A lot of the drawings I did during this period were just straight up copies of photographs of animals from books, because at the time I considered art to be a game of memorization; and that the more images you had in your head and under your fingers then the broader your palette would be when it came time to just make stuff up.
I thought at the time that that's how professionals were so great, they'd just spent all this time soaking up images from study that they could just make up anything on the spot; I didn't learn until later that most concept artists tend to do research and collect lots of reference for each piece they do.