Some strategies might include:
1. using extra large zippers
2.using a zipper that has 2 different color sides
3. Use zipping boards as well as zipping large clothing and bags
4. Pull a pipe cleaner downward through the zipper slot to practice that motion.
There is a picture of the last one on this page: http://www.therapystreetforkids.com/r-buttons.html
I made the following practice activity out of a baby wipe container. The edges have a curve to them. The white sides shown below are cut from the box's edges. The 2 pieces are cut from 2 sides and connected by overlapping them. I covered the inside with black contact paper to make it easier to discriminate. The ends also have blue tape attached to make it easier to see where they are.
The colorful strip is a piece of plastic with covered with a fruit design contact paper. I punched a hole near one end and attached a small clip that I found, but any small object that is easy to pull can be attached.
Now show the child how to insert the colorful plastic piece and pull upward until it comes out. Its not the same as actually zipping but a nice way to introduce the motion involved in pulling the tab upward.