As you know applications like gmail / yahoomail they constantly keep changing their js/css and when you login you dont have to clear your browser cache to see the new changes, they are automatically shown to you.
How do they do this? I see there are two approaches:
1) in your code where you specify the path of the js/css you append time with a querystring that will force your browser to load the lastest js/css no matter it has been modified or not.
2) You use the "Pragma: No-cache" Tag in your page.
But the problem in these approaches is that your application makes an HTTP request every time its called since it assumes the file is new.