If your PCIe slots will let you, increase the gap between your cards.

Get one of those adjacent PCI slot blowers and stick it next to the hot one.

If your case will let you, add some more intake and/or outflow fans to increase the airflow to all your cards.

Run your case with the side off and point a directional fan at the hotspot.

Put your case in front of your air conditioner (NOT swamp cooler).

Those are the easy options; they all have different costs associated with them, so caveat emptor et cetera.