Um, people, what are you on? Sakura's thanked him twice in part I:
http://www.onemanga.com/Naruto/74/06/
http://www.onemanga.com/Naruto/183/14/
In the first case, she must have had a bad impression about the reaction she'd get when thanking him.
In the second case, she thanked him Before he made the promise of a lifetime! ^^
Apart from that:
- she wanted to safeguard his dreams in the first test (by forfeiting her team)
- she wanted to protect him and the others in the second test
- she cheered him against Kiba in the prelims
- she seemed too worried to cheer for him against Neji, but she did express joy at seeing him win, and thought about how he inspired her
Then she decided to go with an "Actions speak louder than words" approach. She saw Naruto all injured, so she decided to become a medic so that she can support him.

I think I don't need to speak for part II.