I have a small site hosted on Dreamhost.
Yslow Score with default setting is C, or 77.
Later I do the following optimization to increase it to B, or 89. Based on Yslow v2 test.
The key is to change .htaccess file.
Put following content into .htaccess file on root of the site.
The second improvement I made is do not use external domain’s images. Put all images on the same site. It reduce the DNS loolup time.
It took me about 10 minutes to implement this optimization.