Working for myself is still new enough that I want to make note of what works and what doesn't.  Working for several clients at once works because thinking about several challenges on parallel tracks results in a kind of cross-pollination of ideas and insights.  Taking a break from one project by thinking of another assignment diverts my conscious attention from a problem I'm stuck on, allowing new ideas and options to sidle in sideways.  Figuring out one puzzle opens doors to others.  I don't know why creativity begets creativity, but it does, and it's much easier and often downright fun to generate from several sources.