Every once in a while, when you’re working on a large file in Photoshop, you might want to make a trip to Edit>Purge>All – which will free up a LOT of memory. Photoshop stores your history and the clipboard in memory – which takes up quite a bit of space depending on how you work. You may also wish to experiment with lowering the amount of steps that Photoshop saves in your history palette by making a visit to the preferences. The more steps saved, the more RAM used.