Well since I use a mac, I use SkinEdit. But it sucks because the 3d model doesn't work. So I also use novaskin to see how it looks. I don't use novaskin because i think it's to difficult. So pretty much what I do is create the skin on skinedit, then upload that onto novaskin and see if I have any errors. But if you use a PC then use MCskin 3D. It even has an automatic shading thingy which I really wish skinedit has. But yeah, that's how I do it.

this is awesome! Which program you use?