Shoutbox

Hi there, I thought i would come here for help once again as you guys are very helpful.
I have this problem with my website that whenever I update it via ftp and the new parts are up it is not visible to most other people when they view it. They still see the old version which is in most cases fixed with an F5 press. The problem is that the majority of people that view my site do not know about this and therefore I either have to tell them or they just assume the site hasnt been updated.
Is there a way to prevent this so that their browser loads the newest version of the page instead of getting it from cache or whatever its doing??
Thanks for your help