Throughout my entire training, from art class at school, to design college and uni, the tutors have always said, "sketch every day!"
It's such a discipline to take your pencils and book out everywhere, and many times, when you finally get the chance to do it, you look around and find yourself on a train or a bus with the most uninspiring interior and odd looks of suspicion if you attempt to draw a fellow commuter.
Well it really doesn't matter what you draw -- the point is that you draw lines or shade or whatever. The point is to work out the muscles in your hand and to practice seeing, to get your hand and your eye to move as one and to produce an impression of the subject rather than a work of art.
“To banish imperfection is to destroy expression, to check exertion, to paralyze vitality.” ― John Ruskin, The Stones of Venice
Ruskin is right. Many times I have been surprised that a scribble which took about fifteen seconds actually has some character and expression. The fast sketches (especially the more you do) seem to grasp something a long study does not. They capture a moment or what your eye observed that second, something that is gone and forgotten forever. This is especially the case when sketching outdoors. Trees, animals and people move, which is a great challenge but can also get you some of those one off moments.
We always sketch best when we find something interesting or inspiring to draw. I love nature and so our park is the best place for me to go. If you can, dig out half an hour on your time off (having energy helps a lot!). Just take a couple of pencils or whatever you like to use and limit sketches to a few seconds each. Don't look at the page and don't take your pencil off the page. Yes you get some very odd results, but there's always specks of gold to be found.