How can I prep a pumpkin for painting? I'm trying to paint a white pumpkin, and I'm having some issues with applying the paint.

I'm using acrylic paint, craft-grade not art-grade, and applying it with brushes. I clean the surface off, first, but nothing else.

The main issue is that the paint applies too thinly, because it would rather stick to the brush than the pumpkin. When applying additional layers, if they aren't sufficiently wet/heavy then the initial layer occasionally rubs off. 

So, for my principal work area I laid down a couple coats of white. This had the unfortunate side-effect of covering up my sketch/under drawing, and it still doesn't resolve the rubbing away issue.
In addition to John's answer, there are some options in the art supply world. In particular, Qor makes a watercolor ground that is good for watercolor, inks, and acrylics. The idea is similar to gesso and provides a base for painting on surfaces not otherwise usable for water-based media.