May I know what error are you getting ? Are you getting a "Page expired" message which tells you to hit the refresh button ? If this is the case, then you may be submitting a form in post method to the previous page. You may be preventing the page to go to cache. In that case you cannot prevent this. This is a browser security feature. You may have to change the submission method of form to Get (if possible).
If the behaviour is not like this, please specify what exactly happening when you hit back button. You may give a piece of code in the page also.