Simply add the following lines to httpd.conf.

    AddOutputFilterByType DEFLATE text/css text/javascript application/x-javascript

or with if condition:

<IfModule mod_deflate.c>
    AddOutputFilterByType DEFLATE text/css text/javascript application/x-javascript
</IfModule>

or with logs:

    AddOutputFilterByType DEFLATE text/css text/javascript application/x-javascript

    DeflateFilterNote Input instream
    DeflateFilterNote Output outstream
    DeflateFilterNote Ratio ratio

    LogFormat '%v %h %l %u %t "%r" %>s %b "%{Referer}i" "%{User-Agent}i" mod_deflate: In:%{instream}n Out:%{outstream}n:%{ratio}npct.' deflate
    CustomLog logs/deflate_log deflate

For more details, please read http://www.clarkconnect.com/wiki/index.php?title=Howtos_-_Activating_content_compression_on_Apache