Tips:
-Use more consistent dithering patterns (I tried random dithering when I was starting out; it just doesn't seem to work very well).
-Cover a larger area with your dithering. Dithered areas should be as wide as the flat coloured areas they are adjacent to.
-Use more contrast (darker darks and lighter lights).
-Use a stronger highlight.
-Think about using more colours. They help you make the transition between shades smoother.

And of course the number one tip: Practice, and keep posting your work and getting feedback. Criticize others' work too.