Adding a schrader valve somewhere on the tank helps to add air without having to completely drain the tank. Especially when you are in a hurry.

And if you want to do it right dougwalkabout's way is the way to go. But remember to only charge the pressure tank with air to slightly less than the pumps cut in pressure. My well has a 20-40 pressure switch on it and pressurizing with air to 30psi means that the pump won't come on till some of the air bleeds out. I hope that made sense.