I've been doing web development for a while and have yet to read a good answer to this question:

Given a page that executes transactions through a postback, how do you prevent the user from duplicating the transaction when they do something as simple as hit the back button? I've explored trying expire the cache but I must admit I'm lacking in my understanding of ASP.NET caching. What is the approach I should use to make the old request stale and invalid? Thanks!