How do I get rid of alot of white space in partitioned table

We have a database that writes 2500 row a minute to a partitioned table. We use partition switching to switch out the old data from the second partition. The data is then dropped at a later date. This activivty is causing the db files to grow; currently the files have about 25% of white space in the data files; the DB is ~ 250GB in size.

How can i manage the size of the db files while using partition switching to purge old data?

See:

serverfault.com/…/is-it-safe-to-have-sql-server-auto-shrink-turned-on