One of the most magical aspects of painting in Photoshop is the History feature. Click Window>History in the top menu bar to open up a descending timeline of your past actions. Every time you paint a line, erase a smudge, make a selection, or do pretty much anything, the History window makes a note of it. Clicking the image at the top of the window will take you all the way back to the file’s original state when you opened it (this is blank if it’s new), while checking a box to the left of any history state will save it so you can go back whenever you need to. This is helpful when you’re happy with how a painting is developing, but want to experiment without fear of ruining anything.