You can occasionally go to your AE RAM folder and delete those files to clear up some space. When you RAM preview in After Effects it creates these files that are temporary. I had 90MB left on my computer and after I deleted my AE RAM I had 200gb. 🙂
When the program no longer lets you RAM preview you need to raise up the number or delete your old files. You can set your Media & Disk Cache in the AE Preferences and adjust your MAX Disk Cache Size. You can also choose which folder it saves the AE RAM too.