Effective use of negative space has always been a challenge for me. It's the idea of defining space and form by creating boundaries; it's allowing the viewer to infer shape and substance from the spaces you left blank. To me, effective use of negative space is a sort of "addition by omission" in the same way that carving or etching is "addition by subtraction."
Here's an example from a master:
Arthur Melville (1858-1904), "Kurrachan"
Here are some examples from some guy with a blog (me):