And how do I do that appropriately?
This depends on the situation and the person who keeps rescheduling. If this is a casual coffee with a friend, who's busy life legitimately gets in the way, then consider “three strikes, you’re out.” If you are trying to network to get a job interview or support, then you might be in a position to overlook their rescheduling before you decide to cancel it all together.
There is another possibility; someone is trying to send you a message; maybe they really do not want to meet. This one is tricky; you have to follow your instincts and pick up on signals. If this is the case, be polite and say, “Maybe it’s best if we cancel for now.” This saves face for everyone, and you move on.
In the case of a busy friend, whom you know would love to meet and catch up, but their life keeps getting in the way—something always seems to come up right before you are supposed to meet—I’d directly, but kindly ask them if life is just too hectic to plan to meet right now. While you’d love to see them, perhaps it’s better if canceled for now and tried again in a few weeks or months. You might be surprised at the relief you could be offering your busy friend. Here, you are really trying to determine that yes, while your friend would love to meet, they simply have too much going on right now. It’s nothing personal, and you can try to meet up again in the future. Don’t we all have a friend like this?
On the other hand, if someone is doing you a favor; a job interview, help with a project you’re working on or any other sort of assistance, then you may need to be a bit more patient. If this person reschedules two or three times, or more, then I think your patience should be in direct proportion to the favor they are doing for you. I am fortunate enough to help with local charity events and when I ask to meet to get help from others, I have learned to be patient and work with their schedules. I am seeking a favor, so their schedule trumps mine.
Everyone is busy these days, and it doesn’t cost a thing to be a bit understanding and work as best you can to coordinate your busy schedule with someone else’s.