Floating divs background color

Hi everyone!!
I got a little problem here that i couldnt resolve…
In my blog, i have a “mainWrapper” div that works as a container for all of my blog content.
Within that div there are another divs that requires to be floated.
Now, here comes the pain:
if i try to put a green backgound color to that “mainwrapper” div, it does nothing. It just shows a little green square caused by the padding i also added to that div, but it doesnt puts a green background under the rest of the content.
I think this is caused by the float property of all the other divs within the wrapper, because if i remove it, the whole thing works great, but sadly i need those divs to be floating…

Another aproach to the solution was to add a float to the wrapper div. If i do it, the background thingie works fine but it breaks the center align of that div (margin: 0 auto;)
any idea??? T_T

thanks in advance!!!