You can simply clear the controls value from the save event itself....
The best way is to clear the controls when showing the modal popup, you can use the
onShowing
event for this purpose
If you want to do by client script only then go for..
OnOkScript
in Modal popupextendar
OnOkScript - Script to run when the modal popup is dismissed with the OkControlID
but make suere that you have captured the value to save.