When your motivation to sketch the human figure is low, turn to one of your sketching heroes/heroines for inspiration. Increase your motivation and interest even more by getting to know who they were (or still are), what they think and write about, and what others have said about them. And most importantly get to know their artistic output by putting your favorites of their works together to see common visual themes inspire you that will in turn help you understand why you love and are so inspired by their art.