How to use same jsp for edit and add form

I would like to use the same .jsp file for edit and add. I just want to change a few controls based on edit/add mode. How do I configure this in the jsp?

In the environment I currently have access to do not have similar setup.
Let say you setup the DFF1 default value to $HEADER.customer_name, the name of the DFF is CUSTOMER_NAME, description can be anything.
In DFF2, default value should be a SQL, where you can reference the DFF1. For example, SELECT DECODE($FLEX.CUSTOMER_NAME, 'ABC', 1, 'DEF', 2, NULL) FROM dual
In this case, you can reference the DFF1.