secure the drywall to stop the cause of the cracking. Locate the trusses/joists and make sure the drywall is screwed securely to them. Run the screws in just enough to be imbedded. Then lightly score the existing crack in a vee fashion about 1/8" wide and about that deep. Using a flexible putty knife, fill the crack and screw heads with all purpose joint compound and let it set a few minutes and then lightly sponge it with a damp sponge to smooth it out and remove excess from surrounding paint. After a day or two of drying, check to see if the compound is nicely bonding and level with the ceiling surface. Sponge sand as necessary to get the area smooth and level.
Then, smear compound on a flat trowel about 1/4 to 3/8" thick and start dabbing it around the repair area and pulling the trowel off to form compound peaks and blending around the existing area. This is where you have to decide if you are getting a good match. Add more mud or spread it out more to get the desired effect. After the compound has set up to approximately half-dried, you can knock the finish down using the trowel pulled very lightly and close to flat accross the finish in the same direction as the original. If you knock it down too soon or tilt the trowel too much you'll have lines in your new finish.
You can sponge the compound off and try again if it's not coming out just right and adjust your method as necessary. When you like it and it's dry, repaint the whole ceiling with a really good paint. Luck, Kizmet