Leverage browser caching and proxy caching by setting far future Expires
headers and changing filenames when file contents change. This can make
your web app faster for users and also reduce your bandwidth costs.

By adding the column :avatar_fingerprint to our db table and including
:fingerprint in the attachment filename, we ensure the filename will
change whenever the file contents do.

  has_attached_file :avatar,
    :styles => { :medium => "300x300>", :thumb => "100x100>" },
    :path => "users/:id/:attachment/:fingerprint-:style.:extension",
    :storage => :s3,
    :s3_headers => {'Expires' => 1.year.from_now.httpdate},
    :s3_credentials => "#{RAILS_ROOT}/config/s3.yml",
    :include_updated_timestamp => false

This enables us to set far future expire headers so that browsers
don't need to check for a newer version. If a change does occur,
say because a user uploads a new avatar, the new filename will
be rendered in your html and the cached version will be ignored.

The example above will set Expires headers in S3. If you're using
local storage you can configure your webserver to do something similar.

We disable the timestamped query string because some proxies refuse
to cache items with query strings.

For more info on optimizing for caching:

http://code.google.com/speed/page-speed/docs/caching.html