My mare gets sort of heat stressed very easily (by the end of a 85-90* day, if she hasn't been hosed off, she'll be acting a little...strange - all cranky, a little "lost", etc) and I've found that adding 4tbls of loose table salt to her feed on hot days (I feed her 1-2T on a daily basis) and really soaking her with water (whether she wants it or not) does wonders.
Since doing those two things, she hasn't had one event of "weirdness" since.
Oh, the other thing I did last summer was, on days over 100*, I stalled her in the shade and wet down+froze her flymasks so she had "frozen flymasks." The only reason I stalled her was that she'd just stand out in the sun and not seek shade, even when it was 111* in the sun. Her stall is shady and has the bonus of a cross breeze blowing through from the deep forest so it stays markedly cooler than anywhere else in the pasture.
Then, I'd let her out at night.