Bring a water bottle or water pack:
For those on the move, you might not be able to just stop whenever you want and grab a nice cup of ice water. Try some of the products over at CamelBak to see what the latest in hydration systems can do for you.
Stay in the shade:
If your run/ride/lift/workout allows it, stick to shaded areas as much as possible. Keeping direct exposure to a minimum will reduce your risk of running dry (literally) and also manages the exposure to UV rays.
Bring a friend:
Going out for a long time? Bring a friend along! Having someone there to talk to or just keep you motivated will make the time go by faster, and if you end up injuring yourself, you've got backup!
Timing is key:
BE smart, and think about when you are going out for your workout. High noon might not be such a great choice; you will fatigue faster, and you run a much higher risk of developing heat sickness or extreme dehydration. Play it safe and shoot for the morning, or in the evening after the highest temps have passed you by.