Typically, I add the rules directly to the Apache config for my whole server (so I don't have to worry about core updates nuking my .htaccess file). Lately, I have been using the rules from HTML5 Boilerplate:

<IfModule mod_deflate.c>
  # Force deflate for mangled headers 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>

  # HTML, TXT, CSS, JavaScript, JSON, XML, HTC:
  <IfModule filter_module>
    FilterDeclare   COMPRESS
    FilterProvider  COMPRESS  DEFLATE resp=Content-Type $text/html
    FilterProvider  COMPRESS  DEFLATE resp=Content-Type $text/css
    FilterProvider  COMPRESS  DEFLATE resp=Content-Type $text/plain
    FilterProvider  COMPRESS  DEFLATE resp=Content-Type $text/xml
    FilterProvider  COMPRESS  DEFLATE resp=Content-Type $text/x-component
    FilterProvider  COMPRESS  DEFLATE resp=Content-Type $application/javascript
    FilterProvider  COMPRESS  DEFLATE resp=Content-Type $application/json
    FilterProvider  COMPRESS  DEFLATE resp=Content-Type $application/xml
    FilterProvider  COMPRESS  DEFLATE resp=Content-Type $application/xhtml+xml
    FilterProvider  COMPRESS  DEFLATE resp=Content-Type $application/rss+xml
    FilterProvider  COMPRESS  DEFLATE resp=Content-Type $application/atom+xml
    FilterProvider  COMPRESS  DEFLATE resp=Content-Type $application/vnd.ms-fontobject
    FilterProvider  COMPRESS  DEFLATE resp=Content-Type $image/svg+xml
    FilterProvider  COMPRESS  DEFLATE resp=Content-Type $image/x-icon
    FilterProvider  COMPRESS  DEFLATE resp=Content-Type $application/x-font-ttf
    FilterProvider  COMPRESS  DEFLATE resp=Content-Type $font/opentype
    FilterChain     COMPRESS
    FilterProtocol  COMPRESS  DEFLATE change=yes;byteranges=no
  </IfModule>

  <IfModule !mod_filter.c>
    # Legacy versions of Apache
    AddOutputFilterByType DEFLATE text/html text/plain text/css application/json
    AddOutputFilterByType DEFLATE application/javascript
    AddOutputFilterByType DEFLATE text/xml application/xml text/x-component
    AddOutputFilterByType DEFLATE application/xhtml+xml application/rss+xml application/atom+xml
    AddOutputFilterByType DEFLATE image/x-icon image/svg+xml application/vnd.ms-fontobject application/x-font-ttf font/opentype
  </IfModule>
</IfModule>

Note that this works on MIME type, and not file extensions, so more respones from Drupal will get compressed.