# Compress all text, html, css, and javascript:
AddOutputFilterByType DEFLATE text/html text/plain text/xml text/css text/javascript

# Or, compress certain file types by extension:
<Files *.html>
SetOutputFilter DEFLATE
</Files>

http://betterexplained.com/articles/how-to-optimize-your-site-with-gzip-compression/

These tweaks improved my Yslow score from F to B.

Force gzip compression for all relevant file types.

htaccess, apache

Apache