How to reduce "other"?

I have had my Macbook for about 4 years now, and I religiously clear out my documents to an external hard drive, but more and more lately I have had "Full Start Up Disk" pop-up. I keep checking my storage and it says 120 GB of other. Today I spent hours restoring my computer then selecting the few things I wanted on here, and not it says 114 GB of other. Now I am totally fed up with this since I can't even save my documents for school. What can I do?