It could be specific content, but it might be something kind of random - which are often the best.
Koosje Koene is one of those. Most of her images are pretty "simple" upon first glance, but you know there wouldn't be any sort of consistent style present if she hadn't done something over and over and over again.
She has some good work on her site and some great video content up on youtube - which you can see below.
There are some great and fun tips for keeping up a consistent sketchbook practice on her channel, and in the below video - which is why it is here, so I would say, check it out.