I'm Dr. Brie Day, a family practice physician at Mercy Waukee Clinic. I wanted to follow up on Dr Nate's reply.
There are several medical conditions that may cause your doctor to advise you to limit your sodium intake, especially high blood pressure, congestive heart failure, and kidney disorders. If you have been advised to follow a low salt diet, then it's best not to increase that salt without consulting with your doctor.
A lot of people have heard about the harmful effects of too much salt, and limit it on their own, which is great. In the extreme heat, if you're limiting it too much, and having excess perspiration, you may get a little low on sodium. Most people think sweat is all salt, but really, it has a lower concentration of salt than your blood. But if you have a low salt diet, sweat profusely, and take in a lot of water (which does not replace salt), then you could set yourself up for hyponatremia, as Dr Nate suggested. While we want you to stay hydrated while outside, using an electrolyte replacement drink like gatorade in addition to free water can help with that as well.

My name is Mindi Callison and I am with Bailing Out Benji. The way I get my dogs to head outside is to make sure that I limit their time, each time they go out. That way they realize that you aren't going out for a long walk or a car ride- you are going out to get their business handled and to get back inside. To help with the accidents, I would start taking the dog out as often as possible (once every two hours) for a brief 5 minutes. Make sure you find shade and stay off of the hot concrete (it burns their pads very quickly). Once the dogs goes, give him/her praise. It is a lot like potty training.

As mentioned before, hydration, hydration, hydration!!
Also be aware of some of the signs of heat related illnesses. The first sign is usually "heat cramps" due to overactivity and dehydration. This progresses in severity to heat exhaustion. Signs include racing heart, headache, lightheadedness, cramps, profuse sweating, weakness, nausea or abdominal cramps. They have an elevated core body temperature (101* - 104*). The next level of severity is heat injury. This is an elevtated body temp with damage to the internal organs (liver, kidney, muscles). And finally there's heat stroke. This is a life threatening condition. The core temp gets above 104*, there is organ damage, and evidence of neurological injury (evidenced by disorientation, confusion, siezures, or coma). The person is hot to the touch, but actually stops sweating.
If you can recognize the signs early on, and get to a cool environment and start rehydrating, you should easily avoid progressing through the different stages of heat illness.

There is no real formula as it depends on your body type and health. Mayo clinic recommends that each male consumes about 13 cups and females consume at least 9 cups. The old 8 x 8 oz glasses is easy to remember; however, it lack evidence supporting its implementation. The Institute of Medicine states that the above recommnendations are appropriate. If you are exercising lightly outside in short bouts, another glass or two should suffice, but if you are working vigorously or outside for long periods of time, additional consumption is required during the activity. It is a must that you continue to drink once you have fnished the event or work.