Adding booster fans to your ceiling A/C vents.

Cool idea!

X2 on what wildtoad said. I spend about two hours with a giant roll of aluminum tape sealing up leaky and poorly installed ductwork. It made a HUGE difference in air flow. For starters, the separation between the return and output air plenums in the main A/C unit was quite porous. Nobody had bothered to tape up the edges of the styrofoam baffles, and most of my cold air was being sucked right back into the intake.