How do you turn off the "Try the new Safari" notification?
You can get rid of it without giving in to Apple‘s desire for you to “Try Now”. Even if you click the button, they will ask you to change your default and may still ask again at a later time. I have explained it in much greater details on my blog but the gist of it are these three commands:

defaults write com.apple.coreservices.uiagent CSUIHasSafariBeenLaunched -bool YES
defaults write com.apple.coreservices.uiagent CSUIRecommendSafariNextNotificationDate -date 2050-01-01T00:00:00Z
defaults write com.apple.coreservices.uiagent CSUILastOSVersionWhereSafariRecommendationWasMade -float 10.99


You must log out and in again for the change to apply. You should actually do it immediately to prevent the system from reverting the changes.

I also cover how to stop Safari from prompting you to become the default every few days. (epimorphic touched on this briefly.) Interesting if you need to use Safari on occasions for site-compatibility, testing, or whatever reason. (Oddly enough, I have not heard any automated browsing test suite authors complaining about this yet.)