According to George Gjoka, a professional Paintless Dent Repair (PDR) technician, when you attempt to take a dent out of your car, you're probably doing it wrong.
You see, when a dent occurs on your car, the metal not only goes inward, but the surrounding edges of the dent go upward, creating a sort of volcano structure. Typically, people go straight to pushing the dent back in from underneath. The thing is, that's not the best way to go about it.
Instead of just popping the dent out from behind, George Gjokaj slowly goes to work on both sides of the dent, using a combination of a long metal rod underneath it and a rubber knock down on the top side in order to completely get the dent out of the car.
You can watch Gjokaj perform his PDR magic on a small dent on the hood of a black Porsche 911 in the video below.
After getting the dent out, all you have to do is sand and buff the area. Just make sure to be careful with this part, as you don't want to sand or buff away too much of your paint job.