You can use a safety pin and a string. Tie the string to the closed end of the safety pin. Open the pin and put it through the hole on the zipper pull. Close the pin and then pull up the string. When you get to the top, take the pin out of the hole. And voila! You are zipped.
Use a latch hook if you have one, they have a long enough handle to make up for reach. Otherwise, if the zipper tops at the neck, slip a string through the zipper hole and pull on an overhead motion. After the dress is zipped slide the string out of the hole.
I personally use a long necklace. It is easy to hook the "claw" part of the clasp into the zipper. Then you simply pull the ring end up and unclasp the claw. We are typically accustomed to clasping and unclasping these behind our necks, so it is a comfortably easy thing to do.