Press CTRL-SHIFT-P to enable Pony Edit Mode. Edit .minecraft/skin_edit.png with an external editor. Whenever you edit this file, your skin will update in-game for preview. Also works with the default (human cuboid) model.
I strongly recommend you to run Minecraft in a small window, keeping it visible alongside your favorite skinning tool. During Pony Edit mode, pressing ALT-Tab in-game will hide the Pause screen, so you can use it with Quick Third Person at the same time.