If your car door is sagging while open, this is because the pins and bushing holding it to the body are worn out. This is a bigger problem than you might think, as the door weighing down on the car can damage other parts in the door or body. A sagging door must be tightened by changing the pins and bushings. You don't need to completely remove the door when removing the old parts, but you do need to keep the door supported upright while you change them.
Place a bench or other platform outside the door. It must be as high as the bottom edge of the door so the door will rest on it while opened and its pins are removed.
Apply masking tape to the door's edge on the hinge side and the edge of the fender right next to it. Cover the masking tape with duct tape. This combo provides a layer of scratch protection that can be removed afterward without leaving residue.
Open the door, making sure it's directly above the bench. Remove the door detent spring, or hold-open spring, from within the door hinge. It's usually located behind the upper hinge bracket. Insert a pry bar up and behind the spring and pry it out.
Drive the door's hinge pins out of their brackets with a punch and hammer. Hit the pins on their pointed ends while grabbing the flathead ends with vise-grip pliers. This means driving the lower pin up from the underside and driving the upper pin down from the top.
Pry the door slightly out of its hinge brackets. You now need someone to hold the door upright, unless you can support the door with a rope or bungee cord strung between the upper door frame and someplace above like a roof truss.
Pound the door pin bushings out of the pin holes on the brackets. Hit the bushings with the punch and hammer while prying them out with the pry bar.
Place the new bushings into the bracket holes. Press them partially into lace with the vise-grip, and then tap them sharply the rest of the way with the punch and hammer. The bushing heads must face up on each bracket's top side and down on the bottom. They go in the door bracket for the lower hinge and the body bracket for the upper.
Align the door brackets and insert the new hinge pins, driving them in place with the punch. The upper hinge pin points upward and the lower one points downward. Place a new locking ring on the groove along the pin's tip, and pound it down with the hammer and pry bar.
Compress the hold-open spring with a spring compressor. The compressor legs must grab the spring on the coils right before the ends. Insert the compressed spring back in its place between the door and detent mechanism, and loosen the compressor screw so the spring will slip into place.
Close the door and remove the tape from the sides.
