It's frustrating that there's no solution to such a widespread problem. The only economical solution I can think of is building another layer of floor in the upstairs unit, but that's a solution that's unavailable to the person trying to solve the problem. And landlords have no incentive for that kind of work because most tenants endure the noise without complaint.
My previous neighbors were very cooperative when I talked to them and they got some rugs and indoor slippers. But the best solution is to just walk lightly. It's second-nature to me to not walk heavily when I'm in small indoor spaces, but a lot of people don't have any awareness of how they walk. I'm also very tall and I think bigger people have a better sense of their physical impact. I'll have small parties here and I'm always interested in who moves gently versus who stomps around my apartment (with apologies to my downstairs neighbor) and it's never the largest people stomping.