Speed – how to remove query strings from static resources

Hi
for speedreasons i need to know how to remove query strings from static resources.
I tried this in my functions.php but that does not help:

function _remove_script_version( $src ){ 
$parts = explode( '?', $src ); 	
return $parts[0]; 
} 
add_filter( 'script_loader_src', '_remove_script_version', 15, 1 ); 
add_filter( 'style_loader_src', '_remove_script_version', 15, 1 );

How to fix that your theme adds parameters to static ressources?

regards marcus