Get yourself a sketchbook, and try drawing it on paper first. Once you get a good B&W sketch, make several copies of them experiment with pencil colors until it "looks about right," and then go to the computer from there.
Whenever I tried making a new model, I always had a hard time remebering what I originally envisioned when I first started (since I has a crappsty memzory). By sketching it out first, I could not only save my thoughts, but I could also take it to a nice, comfy chair. Plus, there are some effects with pencils that are far easier to do freehand than in paint. (Oh, and pencils are erasable)