Writer's Fallout Shelter

I'm sitting here tonight, trying to write a short humor piece when I'm feeling decidedly un-funny. Made me think of the other things we as writers are required to do on-demand, when we might want to do something else. Like articles about summer and summer-activities.

Right now, most of the Western world is gearing up for the winter holidays, but editors are looking for submissions for their summer issues, which means we writers have to be thinking about sweltering heat and swimsuits and vacation destinations and the Fourth of July, instead of Christmas, New Year's, and winter wonderlands.

How do you get yourself in the mood to write something specific when your mind is elsewhere and your body would very much like to follow?