Most of the time I find myself working in series because I couldn't try all the what ifs in one piece of work. Or the work itself brought up a lot of those what ifs and I needed to explore them. But I have on occasion deliberately created a series.
One I'm working on right now is my Fruits of the Spirit angels. In order to not make the series contrived or stiff I've been working on it over a period of time, not forcing the designing, which is what often makes a piece not work. The first angel — Love — was created in 2002 and over the next year I create 2 more — Peace and Faithfulness. Then I got side-tracked as my teaching career took off and didn't seem to have the time or inclination to do any more. About a year ago I was trying to think of a quick project to do to balance the larger projects I was working on when the Fruits popped back into my head. Within a day I had designed the next angel — Joy — and I stitched it over a weekend retreat.
So a planned series can be very successful if the artist restrains herself and lets the work come naturally. Unfortunately this restrained attitude can be short circuited by the demands of a gallery showing or exhibit or deadlines associated with teaching a class. It is often a fine balance between meeting these deadlines and being true to the integrity of the design.