To do this, I sometimes choose a music-driven exercise to get into the rhythm of sketching.
Pick a fresh page in your sketchbook, then queue up and listen to a piece of music, while expressing what you're hearing and feeling on the page.
Don't worry about making it "right" because it's based on how you feel at the moment. Because it's open and free, this can relax you to enjoy the feel of pen or pencil on paper.
This process really helps me focus in on the music, loosens my hand up and gets me in the right frame of mind for sketching.
Above is the sketch created while listening to Impressions by John Coltrane, a wonderful piece which itself helps me loosen up for sketching.
Give it a try sometime!
Music-driven Moleskine Sketches