I usually have a sketchy plan for the blocks, drawn out on graph paper. I'm a scrappy quilter so I take stock of my colors to see how many I can incorporate into an EASY design and just go block by block, then row by row. My plans usually have to be modified as I go -- as careful as I am, my accuracy never seems to be quite as precise as necessary to make anything complicated. If I'm totally honest about it, I'm just happy if I can get the thing properly squared up before binding. Yes, really.
Also, is it just me or do quilts made this way always take WAY more fabric than you expect, but hardly make a dent in the scrap pile at the end? I think my sewing space is kinda "magical" this way. I'm amazed every time at this phenomenon.
I'm ok with this because I just like piecing and sometimes the changes make the finished quilt better than the plan I had to begin with. Not always, though!