How to Trim Database Size?

My problem is about reducing the size of the database which is 6.5TB. Can someone help me on how to trim my database size? My data warehouse system is growing bigger and bigger. It has reached to an average growth of 500GB every month. It takes me 19 full hours for my RMAN to back up and save my data even though I already have enabled BLOCK_CHANGE_TRACKING_FILE. My bitmap indexes is there so it is useless to rebuild my indexes. Please help me. Thanks!
