Jeanne Raffer Beck has put a simple description of what it might mean to work in a series in her latest blog post - http://www.exploringthesurface.com/2008/07/working-in-series-more-thoughts.html
In a previous post she says "Creative minds can be restless, responding continuously to all the visual stimulation that abounds around us -- leading us to jump from one subject and process to another and another. While exploring techniques is richly satisfying, exploring them in relation to a particular theme or subject informs the explorations much more. And the results, rather than leading down a new trail, widen the one we're already on. Everything starts to connect and fit together, sometimes in exciting new ways, sometimes in familiar ones. Even a slight twist can make a huge impact on how the piece "reads", how the work communicates to an audience.
Small changes can be just as significant and interesting as huge, sweeping ones. Perhaps creative people tend to be like kids with their hands in the cookie jar, trying to grab fistfuls and stuff them in their mouths all at once. In contrast, working in a series encourages the artist to savor each bite, allowing a full appreciation of the flavor and texture of each one before moving on to the next."
I like this description a lot and feel that I've begun to move on from the cookie jar mentality - I just haven't got to the savoured small bite part yet!