A List Apart has an article about the value of sketching and outlines the need to just get stuff on paper in a tangible form, rather than fussing about how pretty things look or worrying about “artistic” quality.
When you feel inadequate in your sketching, pause and reconsider your perspective. Don’t worry how well you draw. Instead, think of your sketching as visual thinking, which works regardless of your drawing quality. Ugly gets the job done just fine.