Robert Bonomi says...

1) a large 'barn fan'. venting to the outside. this will help pull the
'inside' temperature down to 'approximately' what it is outside.

2) "water the roof". Intermittantly -- just enough to dampen it, and let
it evaporate off. As long as the ambient humidity isn't excessive,
your water costs are reasonable, and you don't have excessive amounts
of minerals in the water, this can be suprisingly effective. If tap
water quality is suspect, or expensive, build a cistern to capture
the rainwater from the roof, and 'recycle' it.

Water is an interesting idea. I'll have to try that.