There are a couple of methods. Some girls are more stubborn than others.
I have used two methods (though most of the time, i just let them set a clutch of eggs).
1. Placing them alone in a wire bottom cage where the bottom of the cage has air flow. You do this with no bedding, just food and water. That method broke one of my girls in just 24 hours. Some people use this method in a dog carrier or something similar. There's no airflow underneath, but she also isn't provided with bedding of any kind. Also, i've read where folks have had to use this method for quite a bit more than 24 hours (several days and nights) before the hen gave up.
2. Dunking in water. I don't really recommend this if it's very cold where you are. But repeated dunking, several times a day in a bucket of cool water can help cool them off (please keep the head above water; this isn't water torture). You want to make sure that you get their bellies fully wet. I did this with one girl every time she went back to the nest for a couple of days, and it worked. Another girl wasn't hearing it and went on brooding pine shavings.
Just one word of warning, my broody-prone girls never tried to hide their eggs from me until after i tried to break them of being broody.