I use Adobe Illustrator. It's much, much, better than Photoshop for vector drawing, but lacks the easy texturing abilities.
So on one hand, I can draw and trace round things quite well, but things like camo with spots, have to be drawn individually and a gaussian blur applied to each one, which can be a pain...
I work in layers and each major component group has four or five layers as well, so for example I would have from top to bottom:
Fuselage - Rivets: Probably the most boring part, but all those little circles add to the whole deal.
Fuselage - Panel lines and outline: Thin lines normally drawn over a background plan. I do draw every single line, as I find the Trace Tool pretty useless...
Fuselage - Mesh: This is a gradient mesh which gives the thing shape, while allowing the colors and markings to show through.
Fuselage - Markings: I have a lot of Illustrator files of markings and flags, and I just adapt/modify as needed
Fuselage - Colour: The base colour will fill the outline, and then more camo colours are added on top. Blurred edges can be tricky, so I usually add a fat line where needed and just apply the blur to that.
Other parts such as the wing and tailplane will have a shadow layer as well.
My 'normal' amount of layers is between 70 and 90...