Change Start Dates without Changing Due Dates

Has anyone figured out a way to change the default
setting so that the due date doesn't change when you
change a start date? I know you can type in "none" and
then retype the new date, but given how much I manage my
dates, I'd rather not have to go through that everytime.
Generally my due dates are inflexible and my start dates
change all the time. Any suggestions?

(I work freelance and use my task list to manage my
work. As priorities shift, I need to change start dates.)