If SQL Server is running slow and operations are throwing errors due to lack of memory, it is necessary to look into memory issue. If SQL Server is restarted all the cache memory is automatically cleaned up. In production server it is not possible to restart the server. In this scenario following three commands can be very useful.
When executed following three commands will free up memory for SQL Server by cleaning up its cache.
Reference : Pinal Dave (http://blog.SQLAuthority.com)