If you've read this post and decided you want to remember something you've sent on Snapchat to another user, it's doable. Go to your messages and press the Snap or letter for a long time. You will be able to tap 'Delete' in a pop-up menu. The Snap will disappear after verifying it (although the other user will see that you have deleted something). So you're probably safe if the person opened the Snap yet. But there's no way to get it back after you've opened and taken a screenshot.