It was my understanding that this was a basic function of Internet Explorer and Firefox, however it is not doing this with my form. I have a calculator that calculates a persons monthly bill based on a few fields that they fill in. When the person presses the back button, I want to have it to where those values are filled in again.

I guess it requires the user to have auto save fields in their browser turned on, but if there is a way to get this to work I would like to know how.

A demo can be found here.

You can use php to post the variables back to the original form.

On the back button, you currently have some javascript that just takes the user back. You could make this button a form, with hidden form fields with the values that were posted from the cost-calculator.
Then, on your cost-calculator page, check to see if these variables were passed. If they were passed, then set the values of each field with their corresponding variable.

You can add some java script code behind it to run.or some other page link which would prove your task.