In the middle of the summer, heightened temperatures can do a number on your engine and make it susceptible to overheating if your radiator is not doing its job correctly. This also applies year-round - while hotter temps can contribute to this factor, you could also overheat in the dead of winter. It all depends on how well your radiator is working.
The radiator's job is to take hot coolant from the engine and dissipate the heat into the air. Although the radiator's job is cooling, the concept is similar to a household radiator in that it gives off warm air. The difference is that this heat is taken from the coolant, allowing that now-cold liquid to circulate back to the engine where it can continue to fight overheating.
Well, that's the idea anyway. Over time, any number of contaminants - rust, sludge, carbon deposits, etc. - can infect your radiator. This is when you need to perform a radiator flush, a simple procedure that will ultimately go a long way toward helping your engine keep cool.
First, make sure your engine is completely cool before starting. Keep in mind that the radiator cap is likely the hottest part of the engine. Even hours after the car turns off, you can still burn yourself. Always use a rag when removing the cap.
Give the radiator grill a nice scrub before you start, using a brush and some soapy water. Clean this much like you would any other car part - this just gets all the dead bugs and grime off the radiator to prevent future particles from contaminating the part.
You'll need to place a pan underneath the radiator's drain valve to catch any coolant that comes out. Do not drain your coolant directly into the ground. It is highly toxic, but smells very sweet and attracts children and animals. The drain valve should have a simple lever that will open it up. You need to catch the coolant in a pan and then transfer that into a disposable container using a funnel. Your town or state should have guidelines for properly getting rid of this substance once everything's done.
Replace the drain pan, because you'll need it again for this next part. Close the drain valve, then stick your garden hose into the radiator and turn it on. Let it run until the radiator is full, then open the valve and flush the water out. Repeat until the water is clear - and again, dispose of this mixture properly, as it contains coolant residue. Close up the drain valve once you're done.
Now you just need to add coolant - a 50-50 mix of antifreeze and water - directly into the radiator. Some drivers in colder states like to use a bit more antifreeze, but don't go overboard. If in doubt, consult your owner's manual. Most radiators hold about two gallons of liquid, so you can measure this out beforehand and then pour it in.
Lastly, you need to "bleed" the coolant system. This is the simple matter of starting your engine with the radiator cap off. After about 15 minutes of idling, turn the heat on in your car full blast. This is simply getting the coolant circulated and releasing any hot air built up in the part. You might have room for a bit more coolant after the air dissipates.
Finally, clean everything off (careful - it will be extremely hot since you just ran the car) and replace the radiator cap.