I use Paint Tool Sai as well for my presets, and I use the "watercolor" tool to blend markings (it's also similar to the "smudge" tool on Gimp.) and make it a little smoother. You could always add a texture to your preset (Open up a texture in another layer, then set that as multiply and put it on top of the other layers) and/or add some shades and highlights in another layer. That's what I do whenever I make my presets, like this one here:
(http://i.imgur.com/vvR0T8e.png) (http://imgur.com/vvR0T8e)