In the past, I found the following to be very effective on zippers as they get cruddy or remain either opened or zipped for a long period of time:

Metal zippers: Vaseline

Plastic sippers: Silicone spray

After coating the zipper with the above, work the zipper up and down it's entire length a few times. Repeat in 2 weeks.

1977 CJ-7, fiberglass body, AMC 360 w/ headers, DUI ignition, Edelbrock intake and Holley 4150 carb, TF999, Dana 300, 4.56 gears lockers, York air comp, 4" susp lift, 2" body lift, BFG 35" M-T tires, Megashifter, AGR pump & box, REP8000 winch.