How can you help the endangered leatherback turtle?

First, doing whatever you can to reduce ocean pollution, both physical trash and chemicals. This can be done by making sure you throw trash in the trash can (not where it can go down a storm drain) and if you live near a beach, you can gather garbage on the beach to keep it out of the ocean. By buying organic foods (this reduces chemical pollutants like pesticides, herbicides, and synthetic fertilizers).

Second, if you buy seafood, try to pick line caught fish. Line caught fish don't ever accidentally catch turtles (although they can catch birds). Being accidentally caught in nets is a threat to the leatherback sea turtles.

Unfortunately, the primary threat they face is egg harvesting. Many people harvest leatherback sea turtle eggs, especially in Southeast Asia. Unfortunately, I am not aware of a way to prevent that from the US, but there are places I've heard of where people keep a watch on sea turtle nests to protect their eggs from harvesting.