Turn up the heat
Now this may not be what you want to do on a hot day, but if your vehicle is starting to overheat you can head it off by turning on the heater to max temp and max speed. This will help cool your engine down as the heater core is basically another radiator thus by letting hot coolant flow through it and blowing air across it will increase your vehicle's ability to get rid of excess heat. Please roll your windows down and open your sun roof otherwise it is going to get really hot really fast in your car.
Add some Water
Now if your vehicle has overheated please shut it down and let it boil over and cool down. This will prevent you from causing serious damage. Now once it is cooled down (about 30 minutes) open the cap and go find some water to fill it back up with. The water doesn't have to be cold. As I mentioned in a previous post water transfers heat better than coolant so by refilling with water you will be able to increase the cooling capacity.
Fix that leak
If your vehicle leaks coolant some where (sometimes coolant leaks are really hard to find) you should always carry some extra coolant in your trunk or back seat. If you let it get too low your vehicle will loose its ability to stay cool and may overheat. This is the worst kind of overheating as some parts of the engine may be starved for coolant such as the heads. This can be disastrous as you could blow a head gasket, crack the head, or crack the block which may be an engine ending failure. If you don't have some premixed coolant then regular water will work as will those stop leak products.
The good news
The good news is that most gas stations carry products that can help to temporally resolve your overheating issues. Most will sell some automotive products like oil, coolant, stop leak, and gas additives. Also they carry water which you can use in a pinch and/or have a hose and spigot out back. Once you get the vehicle home or to a repair shop get the issue resolved. This may be replacing hoses, a radiator, or a flush and fill. Getting a flush and fill will do wonders if you have a vehicle that has a neglected coolant system, if you regularly exchange your coolant you probably don't need a flush and fill as it will only cost more money.