My first advice is the same as the others, drink water!
It is the best way to stay hydrated. As Silvia said, you can eat fruits that have a lot of water in it such as watermelon or pears.
Try not to ride between 12:00 and 16:00 but if you do make sure to do breaks in the shade and to drink water during effort.
Wear breathing clothes like sports clothes that are made to help cooling your body.
Hope this helps :)
Recommend adding lemon/lemon juice to water. Adding lemon makes the body want to drink the water. Have half gallon bottles for water!! There is also a cloth of some sort that can be placed in water to help you stay cool during the warm summer.
I personally drink Gatorade , we sometimes also give it to the horses because of the elctorlights bue to the heat buring the summer and it doesn't harm them and they love it
Carry water with you every where!! When I am riding I put the water bottle on top of arena rail. That way you can ride by and grab it any time!
During the summer I always take to the barn a bottle of water (with ice) to drink whenever I need. I also take some fruit, like bananas, apples or pears (pears have a lot of water) because sometimes I start to feel dizzy while riding because of the heat.
And the most important thing... SUNCREAM! hahahaha
Hope it helps! ;)