Here are some tips to improve your site’s YSlow grade:

• Turn off Etag.
  Just add the following code to your .htaccess file:

  FileETag None

• Add an Expires header to your web component.
  To add an expires header to images, stylesheets, scripts, Flash, just add the following code to your .htaccess file:

  <IfModule mod_expires.c>
  ExpiresActive On
  ExpiresDefault A600
  ExpiresByType image/x-icon A2592000
  ExpiresByType application/x-javascript A604800
  ExpiresByType text/css A604800
  ExpiresByType image/gif A2592000
  ExpiresByType image/png A2592000
  ExpiresByType image/jpeg A2592000
  ExpiresByType text/plain A86400
  ExpiresByType application/x-shockwave-flash A2592000
  ExpiresByType video/x-flv A2592000
  ExpiresByType application/pdf A2592000
  ExpiresByType text/html A600
  </IfModule>

  # The following list can help you to convert common time intervals into seconds:
  # 300 = 5 minutes
  # 2700 = 45 minutes
  # 3600 = 1 hour
  # 54000 = 15 hours
  # 86400 = 1 day
  # 518400 = 6 days
  # 604800 = 1 week
  # 1814400 = 3 weeks
  # 2419200 = 1 month
  # 26611200 = 11 months
  # 29030400 = 1 year = never expires
  This need apache has installed mod_expires.

• Make fewer HTTP requests
  You can use Joomla Plugin: CssJsCompress to aggregate all JavaScript and CSS files into one file, and use inline image combine all CSS images into a single image file.
• Remove the unnecessary modules/components install from joomla admin panel
• Enable cache for every module used and for the whole site.