Leverage browser caching:
Setting an expiry date or a maximum
age in the HTTP headers for static
resources instructs the browser to
load previously downloaded resources
from local disk rather than over the
network.http://code.google.com/speed/page-speed/docs/caching.html#LeverageBrowserCaching
To set an expiry date or a maximum age in the HTTP headers for static resources
Open IIS manager-> Click your site-> Click the HTTP Response Headers in the home page-> Click the Set Common Headers on the Actions panel -> Check Expire Web Content -> Set After 7 days (as suggested for in the page speed analysis “Specify an expiration at least one week in the future for the following resources”
http://technet.microsoft.com/en-us/library/cc770661%28WS.10%29.aspx