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 [...]