If you want to create sharp lines, try using double sided tape to create the negative for your recessed areas. Bondo up to the line, and then sand down to the tape. This will give you consistent depth and a sharp edge once you have removed the tape. You can see it done in my Reach LT wip. It works really well. I find a flat file is also helpful as is an exacto knife. As far as the actual sanding, a palm/mouse sander is going to be your greatest friend. Get a nice variety of grits, from 60 on down to 200. I generally start at 60 or 80, depending on how much I have to work it down. Then go from 80 to 100, to 120, to 150, and then to 200 or 220. That will get things nice and smooth. I also use sanding blocks and straight sandpaper, depending on the curves, whether convex or concave. So start by building your basic curves with the palm sander, and use a block and straight paper to smooth you small high spots. This will get you a nice, beautiful smooth part.