Whenever you buy something from Humble Bundle, click on Adjust Donation, Custom Amount, then drag the middle slider all the way to the right to give the maximum amount to charity. It doesn't cost you anything extra, but it makes a HUGE difference in how much goes to the charity.

https://gamefaqs.gamespot.com/a/forum/d/d8f558e8.png

With this Bundle, the default donation gives a measly $1 to Children's Miracle Network Hospitals, but if you choose a custom amount, they get $17 out of your $20 purchase!