Ajax code placement in asp pages with master page

I have a master page with one content window for asp pages. I place three
columns in the page using css ( it appears that all the css must be
inherited from the master page). The only way I seem to be able to add the
script manager is by actually adding it to each panel or object in the page.
I would like to add all AJAX references to the content placeholder, but
naturally, this throws an exception. Adding them to the GUI throws all of
the arrangement off, and I am back to coding manually.

Suggestions would be appreciated.

Harry