Roleplaying Tips has posted a two-part series entitled “Hipbone’s Connected to the Thighbone” that offers advice on scenario and story arc creation.
Part one is all about building adventures. It includes some great advice about loosely assigning roles to the PCs: stimulator, foil, detective and driver. (Even if you skim the article, read this section — it’s excellent.)
Part two opens with advice on tying the various elements of your adventure into a unified whole, and then moves into story arcs, finishing with a brief note about extending this technique to campaign creation.
Adventure creation is an area where I can just never get enough help — it’s something I’ve always struggled with as a GM. Even if this is one of your strengths, though, you might find some surprises in these two articles.