Design using major & minor breakpoints
Stephanie Rieger:
Major breakpoints are set using media queries in the document head. The minor breakpoints live within those 2-4 style sheets and provide (mostly) the tweaks needed to remove awkwardness. This in effect creates media queries within media queries and provides huge flexibility while keeping it clear why each breakpoint has been set.
One of many good pieces of advice from Stephanie’s On designing content-out.