Shared disk is almost full!!!

We have a 18G SCSI drive that functions as client users’ public drive. However, the disk is about 80% full. Users have all kinds of folders/files and they were told before to archive their stuff coz they can’t just keep filling up disk. “My stuff isimportant and I cannot get rid of it..they’re all important!”. How do you usually overcome this type of issue? Keep buying more drives?? Any good technique will be appreciated.