Something that helps measurably is to use a camelback or other dromedary, or a sports bottle in a neoprene koozie, kept on you or your machine. This applies more to commercial work where you aren't by the truck every twenty minutes or so:
Camelbacks are hot to wear most days but on the days you can bear them, they really help keep you hydrated. Fasten one to your machine if you don't want to wear it. The point is to take small sips regularly. Don't wait until you need water or have a chance to get back to the truck. Make a habit to take two,three passes and then take a couple swigs, mow some more and repeat. If you're pruning /trimming fasten one to your ladder or keep a bottle by your tarp, barrels, generator, whatever....but not kept all the way back at the truck. When you take a step back to check your work take a sip-when you move your ladder take a sip. Try it you will really notice a difference in your energy and stamina through the heat.