Sometimes, the most simple and obvious tips are the best. Everybody knows about them because they're usually based on good old common sense, yet, we usually forget about them. One such tip is to periodically clean up the Downloads folder of your Mac, which, depending on your setup and habits, could quickly go out of control.
The Downloads folder is the default location where any file you download from the internet is saved. Downloading the PDF of your bank statement? It's downloaded to the Downloads folder. Downloading a song from an unofficial source? It's most likely going to the Downloads folder.