HTML5 Boilerplate ( link ) oferece o que parece ser a melhor e mais eficaz solução, junto com muitos outros, como armazenamento em cache, tipos mime etc. Altamente recomendado.
<IfModule mod_deflate.c>
# Force compression for mangled headers.
# http://developer.yahoo.com/blogs/ydn/posts/2010/12/pushing-beyond-gzipping
<IfModule mod_setenvif.c>
<IfModule mod_headers.c>
SetEnvIfNoCase ^(Accept-EncodXng|X-cept-Encoding|X{15}|~{15}|-{15})$ ^((gzip|deflate)\s*,?\s*)+|[X~-]{4,13}$ HAVE_Accept-Encoding
RequestHeader append Accept-Encoding "gzip,deflate" env=HAVE_Accept-Encoding
</IfModule>
</IfModule>
# Compress all output labeled with one of the following MIME-types
# (for Apache versions below 2.3.7, you don't need to enable 'mod_filter'
# and can remove the '<IfModule mod_filter.c>' and '</IfModule>' lines
# as 'AddOutputFilterByType' is still in the core directives).
<IfModule mod_filter.c>
AddOutputFilterByType DEFLATE application/atom+xml \
application/javascript \
application/json \
application/rss+xml \
application/vnd.ms-fontobject \
application/x-font-ttf \
application/x-web-app-manifest+json \
application/xhtml+xml \
application/xml \
font/opentype \
image/svg+xml \
image/x-icon \
text/css \
text/html \
text/plain \
text/x-component \
text/xml
</IfModule>
</IfModule>
EDIT: Uma vez que esta pergunta e resposta ficar recebendo upvoted depois de alguns anos eu estou colocando o servidor H5BP configs link para mais completo otimização .
EDIT: link fixo para link