Prop a 4-by-4 post or similar sturdy piece of lumber or timber between the edge of the deck’s rim joist -- the joist farthest from your house -- and the top of a patio stone or concrete block, so you can safely work under the deck. Cut the post to fit, if necessary.
Double-check that the deck ledger, the supporting joist nearest the house, is firmly attached to the side of your house by examining the underside of the deck. Tighten if necessary the giant lag bolts that hold the ledger to the rim joist that tops your foundation.
Nail any loose galvanized nails in the joist hangers that fasten the joists to the ledger. Add L brackets as additional fastenings between the joists and the ledger, especially if you need to significantly pivot up the far end of the deck -- the area farthest from the house at the end joist.
Diagnose the root cause of the sag -- posts sunk into the soil, sagging joists, rotten beams or the like -- and gather the needed concrete, forms, lumber and fastenings to effect a repair once the deck is lifted.
Run a string from the high end of the deck to the sagging end, and past the sagging end to a stake in the ground. Add a string bubble level to the string, and temporarily tie the string to the post so it's level as indicated by the bubble level. Measure down the stake 1 inch for every 10 feet of deck to create a slope to shed rain. Retie the string securely to indicate where to lift the deck.
Place a hydraulic or “bottle” jack under the rim joist on a pair of 2-by-10 pieces of lumber at least 3 feet long. Place a square of steel plate or a scrap of lumber at least 2 inches thick between the top of the jack and the joist. Lift a short deck by touching the steel plate directly on the rim joist -- and a taller deck by placing a 4-by-4 post of suitable length on top of the plate to reach to the rim joist.
Pump the jack slowly with short strokes to raise the joist. Have an assistant cut and place a shoring joist or joists -- one pair or more per beam -- in place to hold the deck as you make your repairs.