Well I started out doing full customs and only did a few splices and edits. I eventually learned tricks and got better from criticism, looking at sprites and pictures zoomed in, and just learning from old mistakes.
One trick I learned that I think would be helpful to you is comparing your outline to existing mugs. In paint, you can select the background as your second color, then set your selection as transparent so you can drag your outline over other mugs with similar poses to get proportions and stuff right.
Another thing is trying your best not to pillow shade. Even if you don't shade properly, try your best not to pillow shade, because you'll eventually figure out what works, but if you keep pillow shading you'll get it wrong every time. When I started out, I used Asvel's tutorials. This isn't the exact one I used, but hopefully it can help you the same. http://img.photobucket.com/albums/v218/cardelocke/mug_spriting.png
As a sidenote, you generally want to avoid using the outline color on anything but the chin, eyebrows (with darker skin colors so that it blends well), and just a few pixels at the top of the eye.