Open the shed door so you have access to the hinges. Most shed doors use a standard butt hinge. Butt hinges have two leaves that join together with a hinge pin.
Remove the center securing screw that holds each hinge to the shed door opening frame, using a cordless screwdriver and appropriate bit. Loosen the remaining two screws on the center hinge only.
Drive a 4-inch-long wood screw into the center mounting hole of the center hinge, using the cordless screwdriver. The 4-inch-long screws get a better grip into the framing studs of the shed door opening. Tighten the remaining two screws.
Repeat the process for the remaining two hinges, loosening the upper and lower mounting screws and driving a 4-inch screw in the center of the hinge. Remember to tighten all screws, and check the screws securing the hinge to the door.
Measure from the inside left-bottom corner of the door to the inside right-upper corner of the door with a tape measure. Cut a piece of 1-by-4-inch board to the measurement, using a circular saw.
Secure the board between the two corners with 2-inch wood screws. This helps prevent the door from sagging.