You can also run a RCLSTG (option *ALL), but be sure to run an entire system save before. It could take many hours. I recommend this sequence Entire system save, rclstg and IPL at least every month. But first check what Mr. Gilbert said, this is the first step you have to do.
Make sure you have a backup of anything that you’re going to delete !
You can try deleteing journal receivers, history log files (file QHST* in QSYS). You can also try reorganizing files with deleted records (check with someone that knows how your software works first – some software needs to keep the data in the same sequence).
It might be a good idea to schedule the system cleanup job to automatically delete old spooled files and journal receivers, etc – GO CLEANUP allows you to schedule this.
An IPL can sometimes recover unused space.