Silverlight: how to force browser to download updated client version?

I'm working with silverlight and i get this problem:
I build and deploy a new release of the xap to the server by the client still accesssing to a older version of the xap. So i think: clear the browser cache but don´t work and i can't say to every one "Hey clear the cache before access this page" :D

Solution:
In the page you call the xap
change the param  name="source" value="ClientBin\yourxapfile.xap"
to this: (sorry i have post an image because the blogger removes the code)