Personally I can't wear a camelback while I'm riding because it exacerbates neck and shoulder pain and having that thing on my back makes me way too hot. Instead I use 2 water bottles when I'm riding. I plan my rides so that there will be someplace to refill them long before they are both empty. I don't like to ride more than 25 miles without a rest stop anyway.

I generally don't have much trouble with hot weather in the morning or late afternoon/evening. My problem is between noon and 3:00 when the sun is hottest. It's harder to find shady spots when the sun is at it's high point and the roads turn into blast furnaces with the heat reflecting off them. So I do my best to avoid riding during that part of the day when it's super hot out. If I do get caught out in early afternoon I look for shady areas where I can stop and rest, and if needed I take off my helmet and pour water over my head to help cool down.

Also for me clothing color is extremely important. I have to wear light colors to reflect the sun. I cannot understand how people can wear dark colors when they're riding on a hot sunny day. Same for helmets - I avoid dark ones.