I am sorry, but I forgot a few important information.
The side is http://www.blog-it-solutions.de and the function "Caching" in the plugin "wp-super-cache" is active. I also modified the .htaccess file in the right way. Furthermore I use mod_rewrite for caching the side.
If I try to scale my blog on http://pagespeed.googlelabs.com/ I get the message, that I should active browser caching to improve my website performance. The current result is 53 points of 100.
But I think browser caching is on, because I use the plugin wp-super-cache. Do I make an error in my plugin configuration or is the result on Google Page Speed common?