A difference between a weekend painter and a professional artist is usually that most weekend painters don't have a solid body of work while the established artist does.
A body of work is comprised of multiple pieces that are similar in nature. The definition of a body of work varies from artist to
artist. For some artist's, a body of work might be defined by size. For other artist's, it might be color, media, or subject matter.
An artist that is building a body of work should start by creating a distinctive style and build up on the quantity of works as well as quality. You can't just do a couple pieces in a certain media, style or subject matter, you need to have enough for a solid exhibition. There is no exact
number of pieces that constitutes a body of work, but I would think that most exhibits have at least 10 or more pieces to represent a quality showing.
Think about how famous artists have exhibits and books where there is a general theme and style, for instance, take one of my favorite artist Andrew Wyeth's Helga series or his Wyeth at Kuerners series.
To produce a body of work, don't feel like you have to reinvent yourself, work on painting and creating a steady stream of work and then look for the common thread in those works and build it up to make your body of work.
Most importantly, create a steady stream of work, and paint, paint, paint!