hehe, pci extenders is probably best, but...
if you have your mining rig in a regular case (like I do) you might want to try stuffing something inbetween - I've used a plastic bottle cap, but I've heard styrofoam peanuts work great - to give enough of a gap to let the fan breathe.
that's the easiest, but probably not the safest (no idea what the long term effects of basically bending your card's pcb might be)
one thing you can try, depending on your card is take a close look at the cooling solution of your specific card. if it's a reference design with the blower on one end, then you probably need to keep the plastic shroud that covers the heatsink...
if you have a card with a custom cooler, that plastic cover might be doing more harm than good.
I can only speak about the HIS 6950 1GB from experience, but on my card, the plastic cover does pretty much nothing except choke the fan. with the cover on (and no spacer) the cards are almost touching.
so I took the cover off for a closer look and underneath you can see the heatsink has it's own shroud to force the air across the fins.
I removed the plastic cover and now the card has enough room to stay cool (maybe only 3 or 4 millimeters, but seems to be just enough to be able to suck a decent amount of air)
but take a careful look before you try. that plastic cover might be vital to direct the airflow over the fins of the heatsink.