Caching

I started caching some of the static files on my application, I was wondering - Lets say I have an article on my website and I *want* to cache it. How will I cache it AND will be able to make my visitors "re-cache" it if it has been changed? I read how to do that on static pages, but couldn't find any reference that does not use mod_expires (since I don't have it)... And, if I am already sending a message, How do I use *Header set Expires* (on htaccess) and specifying "in a week"? Couldn't find any answer to this question as well..