Dental floss. Thread a length of it through the hole in the pull of the zipper and then use it to pull the zipper up. It effectively makes your arm/reach a lot longer. Of course, if you have one of those zippers which doesn't have a hole in the pull (ie, an invisible zipper), then it's a bit more difficult.
In my experience, however, nobody minds being asked for help to zip up a dress. We're all human.
if you have a flexible arm and back, you can kinda contort yourself to be able to reach and pull up the zipper by yourself (i am definitely someone who does this a lot). but it's not practical for everyone.
the dental floss method suggested above works very well as well.
It's better to just ask for assistance. Zippers can be finicky and break or get stuck easily. I've never met anyone who would not assist when needed and I've been an assisted and an assist-ee!