Swaddle!
This little girl will sleep about twice as long in her carseat when I have her all wrapped up in her swaddle.  Even though she is sopping wet with sweat when I get her out, she loves her little, hot and tight cocoon.  (And the interesting thing is that Scout loved the cool and brisk air when he was her age.  I used to put him to bed with the windows open, and he would have his best sleeps.)
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