mod_expires – This module controls the setting of the Expires HTTP header in server responses. The expiration date can set to be relative to either the time the source file was last modified, or to the time of the client access.

The Expires HTTP header is an instruction to the client about the document’s validity and persistence. If cached, the document may be fetched from the cache rather than from the source until this time has passed. After that, the cache copy is considered “expired” and invalid, and a new copy must be obtained from the source.

mod_expires is included in most default installs.

Check wheter LoadModule expires_module modules/mod_expires.so is enabled or not in httpd.conf, if its not then uncomment it removing the # prefix.

And then add following code into your httpd.conf configuration file -

Restart apache server.

Resource links -

http://blog.cherouvim.com/mod_expires-and-cache-killers/
http://httpd.apache.org/docs/1.3/mod/mod_expires.html
http://www.david-guerrero.com/papers/squid/squid.htm
http://linuxreviews.org/webdesign/602_Apache_mod_expire/
http://jeremy.zawodny.com/blog/archives/009272.html