[Typo3] Syncing testing and production environment

[Elias Humbolt]

I have a testing and a production environment. 

I prepare the things in testing and upload them to the production environment 
as far as templates, extension files, css are concerned. however, somebody is 
already putting content into the production environment. 

How can I most easily sync templates, css, extension from testing to 
production without destroying any content?

elias