Make sure to backup before whatever you do.
So if you are a person who records videos and takes tons of photos with iPhone or whatever the device may be, you eventually upload on your Mac or iCloud. Obviously, iCloud is not a free service and the default option is not enough disk space to store your movies/pictures over time so you end up storing your data locally. If that's the case, you will eventually end up using up your flash memory.
Here is what you want to do.
1. Analyze what is taking up space from looking at your storage usage from your Apple menu --> About This Mac ---> Storage. (Usually movies and pictures are taking up majority of your space.)
2. Use external hard drive to save your Photos.app or iPhoto.app data. Library files will keep growing as you save movies/pictures to these applications. You would want to have these files destined to your external drives.
3. du -s ~/* | sort - n
Go in detail to see which files are taking up space in your machine. If they are other image for movie files, try to move them to an external drive without interfering with your application.
4. This is probably the most unexpected part in saving your storage. When you done 3., you may have recognized your ~/Library/Application Support/MobileSync/Backup or "backup" in your storage usage is taking up quite a space. This is the backup you made from your iTunes for your iPhone. All the past backups are kept unless you have not erased any of the data from your iTunes.
% ls -l ./Library/Application\ Support/MobileSync/Backup/
This actually freed about 30GB for me since it would have saved all the media files in my iPhone as part of the backup. What an unexpected clean up to save the day!!