This CNN article claims that there's very little even China can do to influence North Korea. I wonder why that is, because (though I'm relatively uninformed) it seems to me China could have a big impact on North Korea by reforming their policy for handling North Korean escapees.
Currently Chinese policy is that any escapees are returned to North Korea. This is inhumane because those returnees are jailed and often tortured, not to mention the hardships endured by escapees trying to reach South Korea traveling in secret. I understand that China doesn't want to host a flood of North Korean refugees, but I don't know why they couldn't coordinate with South Korea to efficiently ship any and all North Korean escapees to South Korea. (Maybe South Korea doesn't want this, but they could be made to accept it.) This would probably lead to people pouring out of North Korea, which would put significant pressure on the regime. Not only would this give political leverage, but it's also a far more humane policy.