Breaking in a new tablet. I always seem to have to go through four or five pages of crapish sketches before I get something decent, no matter how 'warmed up' I am. It's similar to the intimidation of the blank page- just with more pages.
Speaking of more pages: perhaps the single most helpful piece of advice I can give someone trying to learn to draw is to use up a lot of paper. I almost wish it was more complicated, but it's simple, and hard. Start sketching. When something feels frustrating or stale, get a new sheet of paper. ( Don't throw out the old ones, reworking partial ideas is often a great warm-up.) But if you're sketching, momentum is more important than trying to make a complete picture on your first or fifteenth try.