For quite a few conjurations (Grease, Web, Stinking Cloud), the Spell Focus or Greater Spell Focus increases the duration of the spell. However most of these spells are AOEs which are subject to a decreasing DC of -1 every round they last.
My suggestion is that, GSF Conjuration (and perhaps even spell school for wizards) would decrease the rate of DC decay by half.
Instead of -1 every round, it'd be -1 every 2 rounds. This could replace the doubled duration of the spell, even.