I'd do early morning (or evening) walks, although around here as the summer gets warmer evenings won't be that cool. The one thing I used last summer a lot when working in the garden was a cooling bandana.
http://www.leevalley.com/garden/page...,42407&p=10286
It's a Canadian company, but looks like the product is made in the US, so you should be able to find something like it. You basically soak it in water, the crystals swell and you tie it around your neck. The guys I worked with (in manufacturing plants) used to swear by them, and they really do seem to keep you cooler. Not that you won't sweat, but I never get that dizzy feeling with the heat when working in the garden.