I bought an oscillating spindle sander and never looked back. You can pick them up for around $100. It is my favorite tool - this coming from a guy who still uses a 7" coping saw to cut out his risers...
When I sanded fadeouts by hand I took the time to make a template that is the exact mirror opposite of the piece that I was sanding. This enabled the pressure to be consistent across the entire surface area that you are working. I did this exactly one time, then I ran out and bought the spindle sander (FWIW). You can make this template in the same manner that you made your bow form, the key is to ensure that it is perfectly square. The other key is to remember that sandpaper is designed to cut so you can't force it. When it stops cutting and you feel the need to apply more pressure you need to get a new piece.