One of the best things you can do is actually your habit, not anything with the actual Deck itself.
The Deck cases come with a pair of zippers.
To remove your possibility of accidentally moving the case while it's open and having your Deck tumble out (which has been done by a ton of people), get into the habit of closing it by meeting both zippers in the middle.
Once you are in that habit, that serves as your visual confirmation of the Deck case being definitively closed, as there's really no way to meet those zippers in the middle while the case is still open.
About to pick up your case, but don't see the twin zippers in the middle? STOP. Confirm that the case is closed before you pick it up and move it.
I know a lot of people recommend the "Deck Lifesaver," but it's really not needed as long as you properly use the built-in features of the case.