It's great to have milk that keeps a long time, but that cream sure can get hard to re-incorporate and this makes the milk much less appealing. Shaking the milk vigorously is very damaging to it and doesn't even do that great of a job, but my husband came up with a really clever solution.
When you're finally ready to use that older milk, just gently tip it back and forth a few times to get things moving a bit, then put it back in the fridge upside down for an hour or so. The milk actually dissolves the cream on its way back up to the top! When it seems ready, just tip it back and forth a few times, then serve as usual, right side up, of course.