Start off with really quick and dirty sprites like the above. Keep programming until something about a sprite really bothers you (for instance, that the outline of the villain above is broken at the edges or that the block lend themselves to poor collision). Then fix that part. While you're in the pixel editor, mess around with it--give the blue guy an awesome top hat, even if the rest of him isn't awesome enough to match. Add a gradient to the block. That way you'll actually learn the practice, rather than leaving in the placeholders because they're good enough.
That's what I did, and look at me now! I'm a world-famous lexicographer with my own private submarine.