a better solution (can even be done in GUI if you don't want write code :-D):

- add a new file to the filegroup

- run DBCC ShrinkFile on the original file with the EMPTYFILE-Option -> this will move all (used) pages from the original file to the new one

- now you can either drop the original file

- or (if you prefer to still use it, e.g. because DBCC reported that it could not move all data) run a usual SHRINKFILE on the original again (because the filesize will not be reduced if DBCC gives this error)

- run DBCC ShrinkFile with EMPTYFILE on the new file -> moves the data back to the original file

- drop the new file

- repeat if you have more than one filegroup and have to shrink the other ones too

Used this methode few minutes ago to shink my msdb from about 2 GB to 400 MB