setting default value

how can i set a default value in fields from any other tables .while iam using the following code
the value is stored in the table but my need is the value should not stored but it should be just visible. and it will store when i press the save button pls give some idea .
Thanks & Regards

Please clarify :

You want to set a default value while creating a new record or in an existing record??

hi ,

yes, but it should not saved automatically it should be saved while clicking the save button



yes pls give me a solution thanks in advance

You didnt clear my doubt about new record or in existing record???

existing record