One thing you could try to do is to go through the Disk Utility, and unmount all of your drives, including any internal ones. Actually, specifically any internal ones.
This has helped me a lot in the past with the same issue. That’s assuming of course that you actually have enough memory. If you really are low on memory than it’s a question of making space by deleting whatever you can. If you have projects that you don’t want to delete but don’t need all the unused media, use the Media Manager option.
Good luck.